Calculating your monthly mortgage payment is a crucial step in the home-buying process. Our specialized Mortgage Payment Calculator helps you estimate not just the principal and interest, but also the total cost of owning a home, including estimated property taxes and insurance.
How the Calculation Works
A standard mortgage payment is comprised of four main components, often abbreviated as PITI:
Principal: The money you borrowed to buy the house.
Interest: The cost of borrowing that money, determined by your interest rate.
Taxes: Property taxes collected by your local government, often bundled into your monthly payment via an escrow account.
Insurance: Homeowners insurance to protect your property against damage, also frequently paid through escrow.
Why Your Down Payment Matters
The size of your down payment significantly impacts your monthly obligations. A larger down payment reduces the principal loan amount, which lowers your monthly payments and the total interest paid over the life of the loan. For example, putting 20% down avoids Private Mortgage Insurance (PMI), further reducing your monthly costs.
Interest Rate Impact
Even a small difference in interest rates can change your monthly payment by hundreds of dollars and your total loan cost by tens of thousands. Use the calculator above to simulate different rate scenarios to see how refinancing or shopping for a better rate could save you money.
The Mortgage Formula
For those interested in the math, the monthly principal and interest payment is calculated using the following standard amortization formula:
M = P [ i(1 + i)^n ] / [ (1 + i)^n – 1 ]
Where M is the total monthly payment, P is the principal loan amount, i is the monthly interest rate, and n is the number of payments over the loan's lifetime.
function calculateMortgage() {
// 1. Get Input Values
var priceInput = document.getElementById('homePrice');
var downInput = document.getElementById('downPayment');
var rateInput = document.getElementById('interestRate');
var termInput = document.getElementById('loanTerm');
var taxInput = document.getElementById('propertyTax');
var insuranceInput = document.getElementById('homeInsurance');
var errorDiv = document.getElementById('mpcError');
var resultDiv = document.getElementById('mpcResult');
// Reset error
errorDiv.style.display = 'none';
resultDiv.style.display = 'none';
// Parse values
var price = parseFloat(priceInput.value);
var down = parseFloat(downInput.value);
var rate = parseFloat(rateInput.value);
var termYears = parseFloat(termInput.value);
var annualTax = parseFloat(taxInput.value);
var annualInsurance = parseFloat(insuranceInput.value);
// 2. Validation Logic
if (isNaN(price) || price <= 0) {
errorDiv.innerText = "Please enter a valid Home Price.";
errorDiv.style.display = 'block';
return;
}
if (isNaN(down) || down < 0) {
down = 0; // default to 0 if empty/invalid
}
if (isNaN(rate) || rate < 0) {
errorDiv.innerText = "Please enter a valid Interest Rate.";
errorDiv.style.display = 'block';
return;
}
if (isNaN(annualTax)) annualTax = 0;
if (isNaN(annualInsurance)) annualInsurance = 0;
// 3. Calculation Logic
var principal = price – down;
if (principal <= 0) {
errorDiv.innerText = "Down payment cannot be greater than or equal to Home Price.";
errorDiv.style.display = 'block';
return;
}
var monthlyRate = (rate / 100) / 12;
var numberOfPayments = termYears * 12;
var monthlyPI = 0;
// Handle 0% interest edge case
if (rate === 0) {
monthlyPI = principal / numberOfPayments;
} else {
// Standard Amortization Formula: M = P[r(1+r)^n/((1+r)^n)-1)]
var numerator = monthlyRate * Math.pow((1 + monthlyRate), numberOfPayments);
var denominator = Math.pow((1 + monthlyRate), numberOfPayments) – 1;
monthlyPI = principal * (numerator / denominator);
}
var monthlyTax = annualTax / 12;
var monthlyInsurance = annualInsurance / 12;
var monthlyTI = monthlyTax + monthlyInsurance;
var totalMonthly = monthlyPI + monthlyTI;
var totalInterest = (monthlyPI * numberOfPayments) – principal;
// Calculate Payoff Date
var today = new Date();
var payoffYear = today.getFullYear() + termYears;
var monthNames = ["Jan", "Feb", "Mar", "Apr", "May", "Jun", "Jul", "Aug", "Sep", "Oct", "Nov", "Dec"];
var payoffMonth = monthNames[today.getMonth()];
var payoffDateStr = payoffMonth + " " + payoffYear;
// 4. Update UI
document.getElementById('valPI').innerText = "$" + monthlyPI.toLocaleString('en-US', {minimumFractionDigits: 2, maximumFractionDigits: 2});
document.getElementById('valTI').innerText = "$" + monthlyTI.toLocaleString('en-US', {minimumFractionDigits: 2, maximumFractionDigits: 2});
document.getElementById('valTotalMonthly').innerText = "$" + totalMonthly.toLocaleString('en-US', {minimumFractionDigits: 2, maximumFractionDigits: 2});
document.getElementById('valTotalInterest').innerText = "$" + totalInterest.toLocaleString('en-US', {minimumFractionDigits: 2, maximumFractionDigits: 2});
document.getElementById('valPayoffDate').innerText = payoffDateStr;
resultDiv.style.display = 'block';
}