Calculating your monthly mortgage payment is the first critical step in the home buying process. This Mortgage Calculator helps you estimate your monthly financial commitment by factoring in principal, interest, taxes, and insurance (often referred to as PITI).
How the Formula Works
The core calculation determines your Principal and Interest (P&I) payment based on the loan amount, interest rate, and term length. The standard amortization formula used is:
M = P [ i(1 + i)^n ] / [ (1 + i)^n – 1 ]
M = Total monthly payment
P = Principal loan amount (Home Price minus Down Payment)
i = Monthly interest rate (Annual rate divided by 12)
n = Number of payments (Loan term in years multiplied by 12)
Factors Influencing Your Payment
While the interest rate is a major factor, don't overlook the "TI" in PITI:
Property Taxes: These are assessed by your local government and usually bundled into your monthly payment by the lender. They can vary significantly by location.
Homeowners Insurance: Lenders require insurance to protect the asset. This cost depends on the home's value, location, and coverage level.
Down Payment: A larger down payment reduces your principal loan amount, which lowers both your monthly payment and the total interest paid over the life of the loan.
Interpreting the Results
Your Total Monthly Payment is the amount you will likely write a check for every month. However, it is important to see the breakdown. In the early years of a standard 30-year fixed mortgage, a large portion of your payment goes toward interest rather than paying down the principal balance. Using this calculator allows you to test different scenarios—such as a 15-year term vs. a 30-year term—to see how much you can save on total interest.
function calculateMortgage() {
// 1. Get Input Values
var homePrice = parseFloat(document.getElementById('homePrice').value);
var downPayment = parseFloat(document.getElementById('downPayment').value);
var interestRate = parseFloat(document.getElementById('interestRate').value);
var loanTerm = parseFloat(document.getElementById('loanTerm').value);
var propertyTaxYearly = parseFloat(document.getElementById('propertyTax').value);
var homeInsuranceYearly = parseFloat(document.getElementById('homeInsurance').value);
// 2. Validate Inputs
if (isNaN(homePrice) || isNaN(downPayment) || isNaN(interestRate) || isNaN(loanTerm) || isNaN(propertyTaxYearly) || isNaN(homeInsuranceYearly)) {
alert("Please enter valid numbers in all fields.");
return;
}
if (downPayment >= homePrice) {
alert("Down payment cannot be greater than or equal to the home price.");
return;
}
// 3. Perform Calculations
var principal = homePrice – downPayment;
var monthlyRate = (interestRate / 100) / 12;
var numberOfPayments = loanTerm * 12;
// Calculate Principal & Interest (Standard Amortization Formula)
var monthlyPI = 0;
if (interestRate === 0) {
monthlyPI = principal / numberOfPayments;
} else {
monthlyPI = principal * ( (monthlyRate * Math.pow(1 + monthlyRate, numberOfPayments)) / (Math.pow(1 + monthlyRate, numberOfPayments) – 1) );
}
var monthlyTax = propertyTaxYearly / 12;
var monthlyInsurance = homeInsuranceYearly / 12;
var totalMonthlyPayment = monthlyPI + monthlyTax + monthlyInsurance;
var totalCostOfLoan = (monthlyPI * numberOfPayments);
var totalInterest = totalCostOfLoan – principal;
// 4. Update UI
document.getElementById('piDisplay').innerText = "$" + monthlyPI.toLocaleString('en-US', {minimumFractionDigits: 2, maximumFractionDigits: 2});
document.getElementById('taxDisplay').innerText = "$" + monthlyTax.toLocaleString('en-US', {minimumFractionDigits: 2, maximumFractionDigits: 2});
document.getElementById('insDisplay').innerText = "$" + monthlyInsurance.toLocaleString('en-US', {minimumFractionDigits: 2, maximumFractionDigits: 2});
document.getElementById('monthlyTotalDisplay').innerText = "$" + totalMonthlyPayment.toLocaleString('en-US', {minimumFractionDigits: 2, maximumFractionDigits: 2});
document.getElementById('loanAmountDisplay').innerText = "$" + principal.toLocaleString('en-US', {minimumFractionDigits: 0, maximumFractionDigits: 0});
document.getElementById('totalInterestDisplay').innerText = "$" + totalInterest.toLocaleString('en-US', {minimumFractionDigits: 0, maximumFractionDigits: 0});
// Show results container
document.getElementById('resultsArea').style.display = "block";
}