Estimate your monthly mortgage payments accurately.
Monthly Principal & Interest:–
Total Loan Amount:–
Total Interest Paid:–
Total Cost of Loan:–
Payoff Date:–
Understanding How Your Mortgage Payment is Calculated
Buying a home is one of the largest financial commitments most people will make in their lifetime. Understanding exactly how your monthly mortgage payment is calculated is crucial for budgeting and financial planning. This calculator uses the standard amortization formula to determine your principal and interest payments based on your home price, down payment, interest rate, and loan term.
Key Factors Affecting Your Mortgage
Home Price: The total purchase price of the property. A higher price increases both your principal balance and the interest you'll pay over time.
Down Payment: The amount of money you pay upfront. A larger down payment reduces the loan amount (principal), which lowers your monthly payment and the total interest paid. Typically, 20% is recommended to avoid Private Mortgage Insurance (PMI).
Interest Rate: The cost of borrowing money expressed as a percentage. Even a small difference in rates (e.g., 0.5%) can significantly impact your monthly payment and add tens of thousands of dollars to the total cost of the loan over 30 years.
Loan Term: The duration of the loan. A 30-year term offers lower monthly payments but results in higher total interest costs compared to a 15-year term.
The Amortization Formula Explained
Your monthly payment is calculated using an amortization formula that ensures the loan is paid off completely by the end of the term. In the early years of a mortgage, a large portion of your payment goes toward interest. As time passes, the portion allocated to interest decreases, and the portion applied to the principal balance increases.
Use this tool to experiment with different scenarios. For example, see how increasing your down payment by $10,000 affects your monthly bill, or how choosing a 15-year term changes your total interest paid.
function calculateMortgage() {
// 1. Get Input Values
var homePriceInput = document.getElementById('mp-home-price');
var downPaymentInput = document.getElementById('mp-down-payment');
var interestRateInput = document.getElementById('mp-interest-rate');
var loanTermInput = document.getElementById('mp-loan-term');
var homePrice = parseFloat(homePriceInput.value);
var downPayment = parseFloat(downPaymentInput.value);
var annualRate = parseFloat(interestRateInput.value);
var years = parseFloat(loanTermInput.value);
// 2. Validate Inputs
if (isNaN(homePrice) || homePrice <= 0) {
alert("Please enter a valid Home Price.");
return;
}
if (isNaN(downPayment) || downPayment < 0) {
downPayment = 0;
}
if (isNaN(annualRate) || annualRate < 0) {
alert("Please enter a valid Interest Rate.");
return;
}
if (isNaN(years) || years <= 0) {
alert("Please enter a valid Loan Term.");
return;
}
// 3. Perform Calculations
var principal = homePrice – downPayment;
if (principal <= 0) {
alert("Down payment cannot be greater than or equal to the Home Price.");
return;
}
var monthlyRate = (annualRate / 100) / 12;
var numberOfPayments = years * 12;
var monthlyPayment = 0;
// Handle 0% interest edge case
if (annualRate === 0) {
monthlyPayment = principal / numberOfPayments;
} else {
// Formula: M = P [ i(1 + i)^n ] / [ (1 + i)^n – 1 ]
var x = Math.pow(1 + monthlyRate, numberOfPayments);
monthlyPayment = principal * ((monthlyRate * x) / (x – 1));
}
var totalPayment = monthlyPayment * numberOfPayments;
var totalInterest = totalPayment – principal;
// Calculate Payoff Date
var today = new Date();
var payoffDate = new Date(today.setMonth(today.getMonth() + numberOfPayments));
var options = { month: 'long', year: 'numeric' };
var dateString = payoffDate.toLocaleDateString('en-US', options);
// 4. Update UI
var formatter = new Intl.NumberFormat('en-US', {
style: 'currency',
currency: 'USD',
minimumFractionDigits: 2
});
document.getElementById('mp-monthly-payment').innerText = formatter.format(monthlyPayment);
document.getElementById('mp-total-loan').innerText = formatter.format(principal);
document.getElementById('mp-total-interest').innerText = formatter.format(totalInterest);
document.getElementById('mp-total-cost').innerText = formatter.format(totalPayment);
document.getElementById('mp-payoff-date').innerText = dateString;
// Show results container
var resultsDiv = document.getElementById('mp-results-area');
resultsDiv.className = "mp-results visible";
}