GuideCalculatorDetailed Guide

How to Build a Mortgage Calculator

A mortgage calculator helps homebuyers estimate monthly payments, total interest, and amortization schedules. It's one of the highest-traffic financial calculators online.

What is a Mortgage Calculator?

Mortgage calculations determine the monthly payment needed to pay off a home loan over a set term, including principal and interest. Additional factors include property taxes, insurance, and PMI.

The Formula

M = P[r(1+r)^n] / [(1+r)^n - 1]
Where: M = monthly payment, P = principal (loan amount), r = monthly interest rate, n = number of payments

Code Example

JavaScript

function calculateMortgagePayment(principal, annualRate, years) {
  const monthlyRate = annualRate / 100 / 12;
  const numPayments = years * 12;

  if (monthlyRate === 0) return principal / numPayments;

  const payment = principal *
    (monthlyRate * Math.pow(1 + monthlyRate, numPayments)) /
    (Math.pow(1 + monthlyRate, numPayments) - 1);

  return payment;
}

function generateAmortizationSchedule(principal, annualRate, years) {
  const monthlyRate = annualRate / 100 / 12;
  const payment = calculateMortgagePayment(principal, annualRate, years);
  let balance = principal;
  const schedule = [];

  for (let month = 1; month <= years * 12; month++) {
    const interest = balance * monthlyRate;
    const principalPaid = payment - interest;
    balance -= principalPaid;
    schedule.push({ month, payment, principal: principalPaid, interest, balance: Math.max(0, balance) });
  }

  return schedule;
}