Calculate your estimated monthly mortgage payments with ease.
Enter the total amount you wish to borrow.
Enter the yearly interest rate for your mortgage.
Enter the total duration of the loan in years.
Your Estimated Monthly Mortgage Payment
$0.00
Total Principal Paid
$0.00
Total Interest Paid
$0.00
Total Cost
$0.00
Formula: M = P [ i(1 + i)^n ] / [ (1 + i)^n – 1]
Where M = Monthly Payment, P = Principal Loan Amount, i = Monthly Interest Rate, n = Total Number of Payments.
Loan Amortization Schedule (First 12 Payments)
Payment #
Payment
Principal
Interest
Balance
Monthly Payment Breakdown (Principal vs. Interest)
What is a Mortgage Payment Calculator?
A mortgage payment calculator is an essential online tool designed to estimate the monthly payments required to repay a home loan. It helps prospective homeowners and existing property owners understand the financial commitment involved in taking out a mortgage. By inputting key details about the loan, such as the principal amount, interest rate, and loan term, the calculator provides an estimated monthly payment, often broken down into principal and interest components. This tool is fundamental for budgeting, financial planning, and comparing different mortgage offers. Understanding your potential mortgage payment is a critical first step in the home-buying process, ensuring you can afford the property you desire without overextending your finances.
Who should use it? Anyone considering buying a home, refinancing an existing mortgage, or simply wanting to understand the cost of homeownership should use a mortgage payment calculator. This includes first-time homebuyers, individuals looking to move to a new property, investors, and those exploring debt consolidation options through home equity. It's also useful for financial advisors and real estate agents assisting clients.
Common misconceptions about mortgage payments include believing the calculated amount is the final, all-inclusive cost of homeownership. Many calculators focus solely on the principal and interest (P&I), neglecting other crucial expenses like property taxes, homeowner's insurance (often bundled as PITI – Principal, Interest, Taxes, and Insurance), private mortgage insurance (PMI), and potential homeowner association (HOA) fees. Another misconception is that interest rates are fixed for the life of the loan; adjustable-rate mortgages (ARMs) can see payments change over time.
Mortgage Payment Calculator Formula and Mathematical Explanation
The core of the mortgage payment calculator lies in the amortization formula, which calculates the fixed periodic payment (M) needed to pay off a loan over a set period. The standard formula is:
M = P [ i(1 + i)^n ] / [ (1 + i)^n – 1]
Let's break down the variables:
Variable
Meaning
Unit
Typical Range
M
Monthly Payment
Currency ($)
Varies based on loan
P
Principal Loan Amount
Currency ($)
$50,000 – $1,000,000+
i
Monthly Interest Rate
Decimal (e.g., 0.05 / 12)
0.001 – 0.0833 (approx. 1% to 10% annual rate)
n
Total Number of Payments
Integer (Loan Term in Years * 12)
120 – 360 (for 10-30 year loans)
Mathematical Explanation:
Calculate Monthly Interest Rate (i): The annual interest rate (APR) provided is divided by 12 to get the rate applied each month. For example, a 6% annual rate becomes 0.06 / 12 = 0.005 monthly.
Calculate Total Number of Payments (n): The loan term in years is multiplied by 12. A 30-year mortgage has 30 * 12 = 360 payments.
Calculate the Annuity Factor: The core of the formula involves calculating the factor [ (1 + i)^n ]. This represents the future value of a series of payments.
Calculate the Payment: The formula then uses this factor to determine the fixed payment (M) that will amortize the principal (P) over 'n' periods at interest rate 'i'. It essentially balances the present value of the loan with the future value of all the payments made.
This formula ensures that over the life of the loan, the sum of all payments exactly covers the principal borrowed plus all the accrued interest. Early payments are heavily weighted towards interest, while later payments focus more on principal repayment.
Practical Examples (Real-World Use Cases)
Let's illustrate with practical scenarios:
Example 1: First-Time Homebuyer
Sarah is buying her first home and needs a mortgage. She finds a property for $400,000 and plans to make a 20% down payment ($80,000), requiring a loan of $320,000. She qualifies for a 30-year fixed-rate mortgage at 6.5% annual interest.
Inputs: Loan Amount (P) = $320,000, Annual Interest Rate = 6.5%, Loan Term = 30 years.
Outputs: Estimated Monthly Payment (P&I) = $2,023.43. Total Principal Paid = $320,000. Total Interest Paid ≈ $408,434.80. Total Cost ≈ $728,434.80.
Interpretation: Sarah's P&I payment will be approximately $2,023.43 per month. Over 30 years, she will pay roughly $408,434.80 in interest, meaning the total cost of the house will be significantly higher than the initial purchase price due to financing costs. This estimate helps her budget for the mortgage alongside taxes, insurance, and other homeownership expenses.
Example 2: Refinancing a Mortgage
John has an existing mortgage of $250,000 remaining on a 15-year loan he took out 5 years ago. The original rate was 7%, and he's considering refinancing to a new 15-year loan at 5.5% to lower his monthly payments and reduce total interest paid.
Inputs: Loan Amount (P) = $250,000, Annual Interest Rate = 5.5%, Loan Term = 15 years.
Outputs: Estimated Monthly Payment (P&I) = $2,141.08. Total Principal Paid = $250,000. Total Interest Paid ≈ $135,394.40. Total Cost ≈ $385,394.40.
Interpretation: By refinancing, John's monthly payment increases slightly from his current payment (which would be higher on the original 7% loan for the remaining term) to $2,141.08. However, the lower interest rate significantly reduces the total interest paid over the life of the loan compared to continuing with the original loan terms. This demonstrates how refinancing can save money long-term, even if the monthly payment isn't drastically lower. He should also consider closing costs associated with refinancing.
How to Use This Mortgage Payment Calculator
Using our mortgage payment calculator is straightforward and designed for clarity:
Enter Loan Amount: Input the total amount you intend to borrow for the property. This is the principal (P) of your loan.
Enter Annual Interest Rate: Provide the Annual Percentage Rate (APR) offered by the lender. Ensure this is the correct rate for your loan type (fixed or initial rate for ARM).
Enter Loan Term: Specify the duration of the loan in years (e.g., 15, 20, 30 years).
Click 'Calculate Payments': The calculator will process your inputs using the standard amortization formula.
How to read results:
Monthly Payment: This is the primary result, showing your estimated monthly payment covering principal and interest (P&I).
Total Principal Paid: This will always equal your initial loan amount.
Total Interest Paid: This shows the total amount of interest you'll pay over the entire loan term.
Total Cost: The sum of the principal and total interest, representing the overall cost of the loan.
Amortization Table: Provides a detailed breakdown of how each payment is allocated to principal and interest, and the remaining balance after each payment for the first 12 months.
Payment Chart: Visually represents the proportion of your monthly payment that goes towards principal versus interest over time.
Decision-making guidance: Use the results to assess affordability. Compare the calculated monthly payment against your budget. If the payment is too high, consider options like a larger down payment, a longer loan term (which increases total interest), or looking for properties within a lower price range. Use the calculator to compare different loan scenarios (e.g., different interest rates or terms) to find the most suitable option for your financial situation. Remember to factor in additional costs like taxes and insurance.
Key Factors That Affect Mortgage Payment Results
Several critical factors influence your mortgage payment calculation:
Loan Principal Amount: The larger the loan amount, the higher your monthly payments and total interest paid will be. This is directly tied to the property's price and your down payment size. A larger down payment reduces the principal, thus lowering the payment.
Annual Interest Rate (APR): This is arguably the most significant factor after the principal. Even small differences in the interest rate can lead to substantial changes in monthly payments and the total interest paid over the loan's life. Higher rates mean higher payments. Refinancing can help secure a lower rate.
Loan Term (Duration): A longer loan term (e.g., 30 years vs. 15 years) results in lower monthly payments because the principal is spread over more payments. However, it also means you'll pay significantly more interest over the life of the loan.
Loan Type (Fixed vs. ARM): Fixed-rate mortgages offer predictable payments for the entire term. Adjustable-Rate Mortgages (ARMs) start with a lower initial rate and payment, but the rate (and payment) can increase or decrease based on market conditions after the fixed period expires.
Points and Fees: Lenders may charge "points" (prepaid interest) or various fees (origination fees, appraisal fees, etc.) at closing. While these don't directly alter the P&I calculation based on the formula, they increase the overall cost of obtaining the loan and can affect the loan's true cost (often reflected in the APR).
Private Mortgage Insurance (PMI): If your down payment is less than 20% on a conventional loan, you'll likely have to pay PMI. This is an additional monthly cost that protects the lender, not you, and is not included in the basic P&I calculation but must be budgeted for.
Property Taxes and Homeowner's Insurance: These are often included in your total monthly housing payment (PITI) but are separate from the principal and interest calculation. They can vary significantly based on location and property value and are subject to change annually.
Frequently Asked Questions (FAQ)
Q1: Does the mortgage payment calculator include taxes and insurance?
A: Typically, standard mortgage payment calculators focus on Principal and Interest (P&I) only. To get your total monthly housing cost (PITI), you need to add estimates for property taxes, homeowner's insurance, and potentially PMI or HOA fees separately.
Q2: What is the difference between the loan amount and the home price?
A: The home price is the total cost of the property. The loan amount is the home price minus your down payment. The mortgage payment is calculated based on the loan amount.
Q3: How does a lower interest rate affect my monthly payment?
A: A lower interest rate significantly reduces your monthly payment and the total interest paid over the life of the loan. Even a small decrease can save you thousands of dollars.
Q4: Should I choose a 15-year or 30-year mortgage?
A: A 15-year mortgage has higher monthly payments but results in less total interest paid and builds equity faster. A 30-year mortgage has lower monthly payments, making it more affordable month-to-month, but you'll pay substantially more interest over time.
Q5: What does 'amortization' mean?
A: Amortization is the process of paying off a debt over time through regular, scheduled payments. Each payment covers both interest accrued and a portion of the principal balance. Early payments are mostly interest; later payments are mostly principal.
Q6: Can I use this calculator for an investment property?
A: Yes, the core calculation for principal and interest remains the same regardless of whether the property is for personal use or investment. However, investment property loans might have different terms, rates, and down payment requirements.
Q7: What if I make extra payments?
A: Making extra payments towards the principal can significantly shorten your loan term and reduce the total interest paid. This calculator shows the standard payment; extra payments would need to be calculated separately or by adjusting the loan term input.
Q8: How accurate is the mortgage payment calculator?
A: This calculator provides a highly accurate estimate for the principal and interest portion of your mortgage payment based on the standard amortization formula. It does not include third-party costs like taxes, insurance, or lender fees, which can vary.
var loanAmountInput = document.getElementById('loanAmount');
var annualInterestRateInput = document.getElementById('annualInterestRate');
var loanTermYearsInput = document.getElementById('loanTermYears');
var monthlyPaymentOutput = document.getElementById('monthlyPayment');
var totalPrincipalOutput = document.getElementById('totalPrincipal');
var totalInterestOutput = document.getElementById('totalInterest');
var totalCostOutput = document.getElementById('totalCost');
var amortizationTableBody = document.getElementById('amortizationTableBody');
var paymentChartCanvas = document.getElementById('paymentChart');
var paymentChartInstance = null; // To hold the chart instance
var loanAmountError = document.getElementById('loanAmountError');
var annualInterestRateError = document.getElementById('annualInterestRateError');
var loanTermYearsError = document.getElementById('loanTermYearsError');
function formatCurrency(amount) {
return "$" + amount.toFixed(2).replace(/\d(?=(\d{3})+\.)/g, '$&,');
}
function formatPercent(rate) {
return rate.toFixed(2) + "%";
}
function validateInput(inputElement, errorElement, min, max, name) {
var value = parseFloat(inputElement.value);
var isValid = true;
if (isNaN(value)) {
errorElement.textContent = name + " must be a number.";
errorElement.style.display = 'block';
isValid = false;
} else if (value max) {
errorElement.textContent = name + " cannot be greater than " + formatCurrency(max) + ".";
errorElement.style.display = 'block';
isValid = false;
} else {
errorElement.textContent = ";
errorElement.style.display = 'none';
}
return isValid;
}
function validateRateInput(inputElement, errorElement, min, max, name) {
var value = parseFloat(inputElement.value);
var isValid = true;
if (isNaN(value)) {
errorElement.textContent = name + " must be a number.";
errorElement.style.display = 'block';
isValid = false;
} else if (value max) {
errorElement.textContent = name + " cannot be greater than " + formatPercent(max) + ".";
errorElement.style.display = 'block';
isValid = false;
} else {
errorElement.textContent = ";
errorElement.style.display = 'none';
}
return isValid;
}
function validateTermInput(inputElement, errorElement, min, max, name) {
var value = parseInt(inputElement.value);
var isValid = true;
if (isNaN(value)) {
errorElement.textContent = name + " must be a whole number.";
errorElement.style.display = 'block';
isValid = false;
} else if (value max) {
errorElement.textContent = name + " cannot be greater than " + max + ".";
errorElement.style.display = 'block';
isValid = false;
} else {
errorElement.textContent = ";
errorElement.style.display = 'none';
}
return isValid;
}
function calculateMortgage() {
var loanAmount = parseFloat(loanAmountInput.value);
var annualInterestRate = parseFloat(annualInterestRateInput.value);
var loanTermYears = parseInt(loanTermYearsInput.value);
var validLoanAmount = validateInput(loanAmountInput, loanAmountError, 0, undefined, "Loan Amount");
var validInterestRate = validateRateInput(annualInterestRateInput, annualInterestRateError, 0, 100, "Annual Interest Rate");
var validLoanTerm = validateTermInput(loanTermYearsInput, loanTermYearsError, 1, undefined, "Loan Term");
if (!validLoanAmount || !validInterestRate || !validLoanTerm) {
document.getElementById('resultsSection').style.display = 'none';
return;
}
var monthlyInterestRate = annualInterestRate / 100 / 12;
var numberOfPayments = loanTermYears * 12;
var monthlyPayment = 0;
var totalPrincipalPaid = loanAmount;
var totalInterestPaid = 0;
var totalCost = 0;
if (monthlyInterestRate > 0) {
monthlyPayment = loanAmount * (monthlyInterestRate * Math.pow(1 + monthlyInterestRate, numberOfPayments)) / (Math.pow(1 + monthlyInterestRate, numberOfPayments) – 1);
} else {
monthlyPayment = loanAmount / numberOfPayments; // Handle 0% interest rate
}
totalInterestPaid = (monthlyPayment * numberOfPayments) – loanAmount;
totalCost = loanAmount + totalInterestPaid;
monthlyPaymentOutput.textContent = formatCurrency(monthlyPayment);
totalPrincipalOutput.textContent = formatCurrency(totalPrincipalPaid);
totalInterestOutput.textContent = formatCurrency(totalInterestPaid);
totalCostOutput.textContent = formatCurrency(totalCost);
document.getElementById('resultsSection').style.display = 'block';
updateAmortizationTable(loanAmount, monthlyInterestRate, numberOfPayments, monthlyPayment);
updateChart(monthlyInterestRate, numberOfPayments, monthlyPayment, loanAmount);
}
function updateAmortizationTable(principal, monthlyRate, numPayments, monthlyPayment) {
var tableHtml = ";
var balance = principal;
var paymentsToDisplay = Math.min(numPayments, 12); // Show first 12 payments
for (var i = 1; i <= paymentsToDisplay; i++) {
var interestPayment = balance * monthlyRate;
var principalPayment = monthlyPayment – interestPayment;
balance -= principalPayment;
if (balance < 0) balance = 0; // Ensure balance doesn't go negative
tableHtml += '
';
tableHtml += '
' + i + '
';
tableHtml += '
' + formatCurrency(monthlyPayment) + '
';
tableHtml += '
' + formatCurrency(principalPayment) + '
';
tableHtml += '
' + formatCurrency(interestPayment) + '
';
tableHtml += '
' + formatCurrency(balance) + '
';
tableHtml += '
';
}
amortizationTableBody.innerHTML = tableHtml;
}
function updateChart(monthlyRate, numPayments, monthlyPayment, initialLoanAmount) {
var ctx = paymentChartCanvas.getContext('2d');
// Destroy previous chart instance if it exists
if (paymentChartInstance) {
paymentChartInstance.destroy();
}
var labels = [];
var principalData = [];
var interestData = [];
var balance = initialLoanAmount;
var totalInterestAccrued = 0;
// Generate data for the entire loan term for accurate representation
for (var i = 1; i <= numPayments; i++) {
labels.push('Month ' + i);
var interestPayment = balance * monthlyRate;
var principalPayment = monthlyPayment – interestPayment;
balance -= principalPayment;
if (balance maxDataPoints) {
var step = Math.ceil(labels.length / maxDataPoints);
labels = labels.filter(function(_, index) { return index % step === 0; });
principalData = principalData.filter(function(_, index) { return index % step === 0; });
interestData = interestData.filter(function(_, index) { return index % step === 0; });
}
paymentChartInstance = new Chart(ctx, {
type: 'bar', // Changed to bar chart for better visualization of breakdown
data: {
labels: labels,
datasets: [{
label: 'Principal Paid',
data: principalData,
backgroundColor: 'rgba(0, 74, 153, 0.6)', // Primary color
borderColor: 'rgba(0, 74, 153, 1)',
borderWidth: 1,
stack: 'Mortgage' // Stack bars
}, {
label: 'Interest Paid',
data: interestData,
backgroundColor: 'rgba(40, 167, 69, 0.6)', // Success color
borderColor: 'rgba(40, 167, 69, 1)',
borderWidth: 1,
stack: 'Mortgage' // Stack bars
}]
},
options: {
responsive: true,
maintainAspectRatio: false,
scales: {
x: {
stacked: true,
title: {
display: true,
text: 'Payment Number'
}
},
y: {
stacked: true,
beginAtZero: true,
title: {
display: true,
text: 'Amount ($)'
},
ticks: {
callback: function(value) {
return formatCurrency(value);
}
}
}
},
plugins: {
tooltip: {
callbacks: {
label: function(context) {
var label = context.dataset.label || ";
if (label) {
label += ': ';
}
if (context.parsed.y !== null) {
label += formatCurrency(context.parsed.y);
}
return label;
}
}
}
}
}
});
}
function resetCalculator() {
loanAmountInput.value = '300000';
annualInterestRateInput.value = '5';
loanTermYearsInput.value = '30';
loanAmountError.textContent = ";
loanAmountError.style.display = 'none';
annualInterestRateError.textContent = ";
annualInterestRateError.style.display = 'none';
loanTermYearsError.textContent = ";
loanTermYearsError.style.display = 'none';
document.getElementById('resultsSection').style.display = 'none';
monthlyPaymentOutput.textContent = '$0.00';
totalPrincipalOutput.textContent = '$0.00';
totalInterestOutput.textContent = '$0.00';
totalCostOutput.textContent = '$0.00';
amortizationTableBody.innerHTML = ";
if (paymentChartInstance) {
paymentChartInstance.destroy();
paymentChartInstance = null;
}
// Optionally redraw an empty canvas or clear it
var ctx = paymentChartCanvas.getContext('2d');
ctx.clearRect(0, 0, paymentChartCanvas.width, paymentChartCanvas.height);
}
function copyResults() {
var monthlyPayment = monthlyPaymentOutput.textContent;
var totalPrincipal = totalPrincipalOutput.textContent;
var totalInterest = totalInterestOutput.textContent;
var totalCost = totalCostOutput.textContent;
var loanAmount = formatCurrency(parseFloat(loanAmountInput.value));
var annualInterestRate = formatPercent(parseFloat(annualInterestRateInput.value));
var loanTermYears = loanTermYearsInput.value + ' years';
var resultsText = "— Mortgage Payment Calculation Results —\n\n";
resultsText += "Key Assumptions:\n";
resultsText += "- Loan Amount: " + loanAmount + "\n";
resultsText += "- Annual Interest Rate: " + annualInterestRate + "\n";
resultsText += "- Loan Term: " + loanTermYears + "\n\n";
resultsText += "Estimated Monthly Payment (P&I): " + monthlyPayment + "\n";
resultsText += "Total Principal Paid: " + totalPrincipal + "\n";
resultsText += "Total Interest Paid: " + totalInterest + "\n";
resultsText += "Total Cost of Loan: " + totalCost + "\n";
resultsText += "\n(Note: This calculation typically excludes taxes, insurance, and other fees.)";
navigator.clipboard.writeText(resultsText).then(function() {
// Optional: Show a confirmation message
alert('Results copied to clipboard!');
}).catch(function(err) {
console.error('Failed to copy results: ', err);
// Fallback for browsers that don't support clipboard API well
prompt("Copy the text below manually:", resultsText);
});
}
// Initial calculation on page load if values are present
document.addEventListener('DOMContentLoaded', function() {
// Add event listeners for real-time updates
loanAmountInput.addEventListener('input', calculateMortgage);
annualInterestRateInput.addEventListener('input', calculateMortgage);
loanTermYearsInput.addEventListener('input', calculateMortgage);
// Perform an initial calculation if inputs have default values
calculateMortgage();
});
// Load Chart.js library dynamically if not already present
// This is a common pattern for external libraries, but for pure JS, we'd implement drawing logic directly.
// Since the prompt requires NO external libraries, we will implement drawing logic directly.
// The Chart.js library is NOT used here. The canvas drawing is done manually or via a simplified internal logic.
// For this example, I'll simulate chart drawing using basic canvas API if Chart.js is not available.
// However, a full native canvas chart implementation is complex.
// Given the constraints, I'll provide a placeholder for a native canvas implementation or a simplified SVG approach.
// For a robust solution without libraries, SVG is often easier for simple charts.
// Let's assume a simplified native canvas drawing for demonstration.
// NOTE: A full native canvas chart implementation is extensive.
// The provided `updateChart` function uses Chart.js syntax for clarity,
// but a true native implementation would involve manual drawing of bars, axes, labels etc.
// For this exercise, I'll keep the Chart.js structure but acknowledge it's an external dependency.
// If strictly NO external libraries means NO Chart.js, then a pure SVG or manual canvas drawing is needed.
// Given the prompt's emphasis on "pure SVG ()" or "Native ", I'll proceed with a placeholder
// that would require a full native implementation.
// Placeholder for native canvas drawing logic if Chart.js is not allowed.
// This section would contain manual canvas drawing commands.
// For now, I'll keep the Chart.js structure as it's a common way to handle charts,
// but acknowledge the constraint violation if Chart.js is considered external.
// If Chart.js is truly forbidden, the `updateChart` function needs a complete rewrite.
// Re-evaluating the prompt: "NO external chart libraries". Chart.js IS an external library.
// I must replace the Chart.js implementation with native canvas drawing or SVG.
// SVG is generally easier for dynamic charts without libraries. Let's switch to SVG.
// — SVG Chart Implementation —
// Remove the canvas element and its script references.
// Add an SVG element instead.
// The following code replaces the canvas part.
// The HTML needs to be updated to include an SVG element.
// For now, I'll comment out the canvas part and add a note.
/*
// — Canvas Chart Implementation (Requires Chart.js library) —
// If Chart.js is NOT allowed, this section needs a complete rewrite using native canvas API or SVG.
// The current `updateChart` function uses Chart.js syntax.
// A full native canvas implementation is complex and beyond a simple script block.
// For a production-ready solution without libraries, SVG is often preferred.
*/
// — SVG Chart Implementation (Replacing Canvas) —
// The HTML needs to be modified to include an element instead of .
// For demonstration purposes, I'll keep the JS structure but note that the HTML needs adjustment.
// The `updateChart` function below is conceptual for SVG.
function updateChart(monthlyRate, numPayments, monthlyPayment, initialLoanAmount) {
var svgContainer = document.getElementById('paymentChartContainer'); // Assuming an SVG container exists
if (!svgContainer) {
console.error("SVG container not found.");
return;
}
svgContainer.innerHTML = "; // Clear previous SVG content
var svgNS = "http://www.w3.org/2000/svg";
var svg = document.createElementNS(svgNS, "svg");
svg.setAttribute('width', '100%');
svg.setAttribute('height', '300');
svg.style.maxWidth = '100%';
svg.style.height = '300px';
var chartWidth = svg.clientWidth || 600; // Default width if not rendered yet
var chartHeight = 300;
var barPadding = 5;
var labelPadding = 20;
var balance = initialLoanAmount;
var principalPayments = [];
var interestPayments = [];
var labels = [];
for (var i = 1; i <= numPayments; i++) {
labels.push('Month ' + i);
var interestPayment = balance * monthlyRate;
var principalPayment = monthlyPayment – interestPayment;
balance -= principalPayment;
if (balance maxDataPoints) {
var step = Math.ceil(labels.length / maxDataPoints);
labels = labels.filter(function(_, index) { return index % step === 0; });
principalPayments = principalPayments.filter(function(_, index) { return index % step === 0; });
interestPayments = interestPayments.filter(function(_, index) { return index % step === 0; });
}
var numBars = labels.length;
var barWidth = (chartWidth – labelPadding * 2 – barPadding * (numBars – 1)) / numBars;
var maxValue = Math.max.apply(null, principalPayments.map(function(p, i) { return p + interestPayments[i]; }));
if (maxValue === 0) maxValue = 1; // Avoid division by zero
var yScale = (chartHeight – labelPadding * 2) / maxValue;
// Draw X-axis labels
labels.forEach(function(label, index) {
var text = document.createElementNS(svgNS, "text");
text.setAttribute('x', labelPadding + index * (barWidth + barPadding) + barWidth / 2);
text.setAttribute('y', chartHeight – labelPadding / 2);
text.setAttribute('text-anchor', 'middle');
text.setAttribute('font-size', '10px');
text.textContent = label;
svg.appendChild(text);
});
// Draw bars
principalPayments.forEach(function(p, index) {
var interest = interestPayments[index];
var totalHeight = (p + interest) * yScale;
var principalHeight = p * yScale;
var interestHeight = interest * yScale;
var xPos = labelPadding + index * (barWidth + barPadding);
// Interest Bar (bottom)
var interestRect = document.createElementNS(svgNS, "rect");
interestRect.setAttribute('x', xPos);
interestRect.setAttribute('y', chartHeight – labelPadding – interestHeight);
interestRect.setAttribute('width', barWidth);
interestRect.setAttribute('height', interestHeight);
interestRect.setAttribute('fill', 'rgba(40, 167, 69, 0.6)'); // Success color
svg.appendChild(interestRect);
// Principal Bar (top)
var principalRect = document.createElementNS(svgNS, "rect");
principalRect.setAttribute('x', xPos);
principalRect.setAttribute('y', chartHeight – labelPadding – interestHeight – principalHeight);
principalRect.setAttribute('width', barWidth);
principalRect.setAttribute('height', principalHeight);
principalRect.setAttribute('fill', 'rgba(0, 74, 153, 0.6)'); // Primary color
svg.appendChild(principalRect);
});
// Add legend
var legend = document.createElementNS(svgNS, "g");
legend.setAttribute('transform', 'translate(10, 10)');
var legendPrincipalRect = document.createElementNS(svgNS, "rect");
legendPrincipalRect.setAttribute('width', '15');
legendPrincipalRect.setAttribute('height', '15');
legendPrincipalRect.setAttribute('fill', 'rgba(0, 74, 153, 0.6)');
legend.appendChild(legendPrincipalRect);
var legendPrincipalText = document.createElementNS(svgNS, "text");
legendPrincipalText.setAttribute('x', 20);
legendPrincipalText.setAttribute('y', 12);
legendPrincipalText.setAttribute('font-size', '12px');
legendPrincipalText.textContent = 'Principal';
legend.appendChild(legendPrincipalText);
var legendInterestRect = document.createElementNS(svgNS, "rect");
legendInterestRect.setAttribute('width', '15');
legendInterestRect.setAttribute('height', '15');
legendInterestRect.setAttribute('y', 20);
legendInterestRect.setAttribute('fill', 'rgba(40, 167, 69, 0.6)');
legend.appendChild(legendInterestRect);
var legendInterestText = document.createElementNS(svgNS, "text");
legendInterestText.setAttribute('x', 20);
legendInterestText.setAttribute('y', 32);
legendInterestText.setAttribute('font-size', '12px');
legendInterestText.textContent = 'Interest';
legend.appendChild(legendInterestText);
svg.appendChild(legend);
svgContainer.appendChild(svg);
}
// Adjust HTML to use SVG instead of Canvas
// The HTML needs to be updated:
// Replace " with:
// ``
// And adjust the caption accordingly.
// The `updateChart` function above assumes this SVG structure.
// For the purpose of this single HTML file output, I will revert to using Canvas
// and assume Chart.js is implicitly allowed or a simplified native drawing is expected.
// Given the strict "NO external chart libraries", a native canvas implementation is required.
// This is complex. I will provide a simplified native canvas drawing that shows basic bars.
// — Revised Native Canvas Implementation —
function updateChart(monthlyRate, numPayments, monthlyPayment, initialLoanAmount) {
var canvas = document.getElementById('paymentChart');
var ctx = canvas.getContext('2d');
ctx.clearRect(0, 0, canvas.width, canvas.height); // Clear previous drawing
var chartHeight = canvas.height – 40; // Leave space for labels
var chartWidth = canvas.width – 60; // Leave space for labels
var barPadding = 5;
var balance = initialLoanAmount;
var principalPayments = [];
var interestPayments = [];
var labels = [];
for (var i = 1; i <= numPayments; i++) {
labels.push('Month ' + i);
var interestPayment = balance * monthlyRate;
var principalPayment = monthlyPayment – interestPayment;
balance -= principalPayment;
if (balance maxDataPoints) {
var step = Math.ceil(labels.length / maxDataPoints);
labels = labels.filter(function(_, index) { return index % step === 0; });
principalPayments = principalPayments.filter(function(_, index) { return index % step === 0; });
interestPayments = interestPayments.filter(function(_, index) { return index % step === 0; });
}
var numBars = labels.length;
var barWidth = (chartWidth – barPadding * (numBars – 1)) / numBars;
var maxValue = Math.max.apply(null, principalPayments.map(function(p, i) { return p + interestPayments[i]; }));
if (maxValue === 0) maxValue = 1; // Avoid division by zero
var yScale = chartHeight / maxValue;
ctx.font = '10px Arial';
ctx.textAlign = 'center';
// Draw X-axis labels
labels.forEach(function(label, index) {
var xPos = 30 + index * (barWidth + barPadding) + barWidth / 2;
ctx.fillText(label, xPos, canvas.height – 10);
});
// Draw bars
principalPayments.forEach(function(p, index) {
var interest = interestPayments[index];
var totalHeight = (p + interest) * yScale;
var principalHeight = p * yScale;
var interestHeight = interest * yScale;
var xPos = 30 + index * (barWidth + barPadding);
// Interest Bar (bottom)
ctx.fillStyle = 'rgba(40, 167, 69, 0.6)'; // Success color
ctx.fillRect(xPos, canvas.height – 40 – interestHeight, barWidth, interestHeight);
// Principal Bar (top)
ctx.fillStyle = 'rgba(0, 74, 153, 0.6)'; // Primary color
ctx.fillRect(xPos, canvas.height – 40 – interestHeight – principalHeight, barWidth, principalHeight);
});
// Add legend manually
ctx.font = '12px Arial';
ctx.textAlign = 'left';
// Legend: Principal
ctx.fillStyle = 'rgba(0, 74, 153, 0.6)';
ctx.fillRect(10, 10, 15, 15);
ctx.fillStyle = '#333';
ctx.fillText('Principal', 30, 22);
// Legend: Interest
ctx.fillStyle = 'rgba(40, 167, 69, 0.6)';
ctx.fillRect(10, 30, 15, 15);
ctx.fillStyle = '#333';
ctx.fillText('Interest', 30, 42);
}
// Initial calculation on page load
document.addEventListener('DOMContentLoaded', function() {
loanAmountInput.addEventListener('input', calculateMortgage);
annualInterestRateInput.addEventListener('input', calculateMortgage);
loanTermYearsInput.addEventListener('input', calculateMortgage);
calculateMortgage(); // Initial calculation
});