A mortgage is a significant financial commitment, and understanding how your monthly payment is calculated is crucial for budgeting and financial planning. This calculator helps you estimate your principal and interest payment based on the loan amount, interest rate, and loan term.
How the Calculation Works
The monthly mortgage payment (P&I) is calculated using the following formula:
M = P [ i(1 + i)^n ] / [ (1 + i)^n – 1]
Where:
M = Your total monthly mortgage payment (Principal & Interest)
P = The principal loan amount (the amount you borrow)
i = Your monthly interest rate. This is calculated by dividing your annual interest rate by 12. For example, if your annual rate is 4%, your monthly rate (i) is 0.04 / 12 = 0.003333.
n = The total number of payments over the loan's lifetime. This is calculated by multiplying the number of years in your loan term by 12. For a 15-year loan, n = 15 * 12 = 180.
Example Calculation:
Let's say you are taking out a mortgage with the following details:
Loan Amount (P): $300,000
Annual Interest Rate: 5.5% (0.055)
Loan Term: 30 Years
First, we calculate the monthly interest rate (i) and the total number of payments (n):
i = 0.055 / 12 = 0.00458333
n = 30 years * 12 months/year = 360 months
Now, plug these values into the formula:
M = 300,000 [ 0.00458333(1 + 0.00458333)^360 ] / [ (1 + 0.00458333)^360 – 1]
M = 300,000 [ 0.00458333 * (1.00458333)^360 ] / [ (1.00458333)^360 – 1]
M = 300,000 [ 0.00458333 * 4.97397 ] / [ 4.97397 – 1]
M = 300,000 [ 0.0228038 ] / [ 3.97397 ]
M = 300,000 * 0.00573848 M ≈ $1,721.54
The estimated monthly principal and interest payment would be approximately $1,721.54.
Important Considerations:
This calculator provides an estimate for the principal and interest portion of your mortgage payment. Your actual total monthly housing expense will likely be higher, as it typically includes:
Property Taxes: Funds set aside for local government taxes.
Homeowner's Insurance: Protection against damage to your home.
Private Mortgage Insurance (PMI): If your down payment is less than 20%, you may need to pay PMI.
Homeowners Association (HOA) Fees: If applicable to your property.
Lenders often include these additional costs in an "escrow" account, and you pay them as part of your total monthly mortgage payment. Always consult with a mortgage professional for personalized advice and exact figures.
function formatCurrency(amount) {
return "$" + amount.toFixed(2).replace(/\d(?=(\d{3})+\.)/g, '$&,');
}
function formatPercent(rate) {
return rate.toFixed(2) + "%";
}
function updateSlider(inputId, sliderId) {
var inputElement = document.getElementById(inputId);
var sliderElement = document.getElementById(sliderId);
var sliderValueElement = document.getElementById(sliderId + 'Value');
if (sliderElement && sliderValueElement) {
var value = parseFloat(inputElement.value);
if (!isNaN(value)) {
sliderElement.value = value;
if (sliderId === 'loanAmountSlider') {
sliderValueElement.textContent = formatCurrency(value);
} else if (sliderId === 'annualInterestRateSlider') {
sliderValueElement.textContent = formatPercent(value);
} else {
sliderValueElement.textContent = value;
}
}
}
}
function calculateMortgage() {
var loanAmountInput = document.getElementById("loanAmount");
var annualInterestRateInput = document.getElementById("annualInterestRate");
var loanTermYearsInput = document.getElementById("loanTermYears");
var loanAmount = parseFloat(loanAmountInput.value);
var annualInterestRate = parseFloat(annualInterestRateInput.value);
var loanTermYears = parseInt(loanTermYearsInput.value);
var resultDiv = document.getElementById("result");
var monthlyPaymentDiv = document.getElementById("monthlyPayment");
var totalPaymentDiv = document.getElementById("totalPayment");
var totalInterestDiv = document.getElementById("totalInterest");
if (isNaN(loanAmount) || isNaN(annualInterestRate) || isNaN(loanTermYears) || loanAmount <= 0 || annualInterestRate < 0 || loanTermYears 0) {
monthlyPayment = loanAmount * (monthlyInterestRate * Math.pow(1 + monthlyInterestRate, numberOfPayments)) / (Math.pow(1 + monthlyInterestRate, numberOfPayments) – 1);
} else {
monthlyPayment = loanAmount / numberOfPayments; // Handle 0% interest rate
}
var totalPayment = monthlyPayment * numberOfPayments;
var totalInterest = totalPayment – loanAmount;
monthlyPaymentDiv.textContent = formatCurrency(monthlyPayment);
totalPaymentDiv.textContent = "Total Paid: " + formatCurrency(totalPayment);
totalInterestDiv.textContent = "Total Interest: " + formatCurrency(totalInterest);
}
// Initialize slider values on page load
document.addEventListener('DOMContentLoaded', function() {
updateSlider('loanAmount', 'loanAmountSlider');
updateSlider('annualInterestRate', 'annualInterestRateSlider');
updateSlider('loanTermYears', 'loanTermYearsSlider');
calculateMortgage(); // Perform initial calculation
});