A mortgage is a significant financial commitment, typically the largest loan most individuals will ever take on.
This calculator helps you understand the impact of making extra payments towards your mortgage principal.
By paying more than your required minimum each month, you can significantly reduce the total interest paid over the life of the loan and shorten the repayment period.
How the Mortgage Calculation Works
The standard monthly mortgage payment (Principal & Interest) is calculated using the following formula:
M = P [ i(1 + i)^n ] / [ (1 + i)^n – 1]
M = Your total monthly mortgage payment (Principal & Interest)
P = The principal loan amount
i = Your monthly interest rate (Annual Rate / 12)
n = The total number of payments over the loan's lifetime (Loan Term in Years * 12)
The Power of Extra Payments
When you make an extra payment, it's applied directly to your loan's principal balance. This has a compounding effect:
Reduced Principal: A lower principal balance means less interest accrues in subsequent months.
Shorter Loan Term: By consistently paying down the principal faster, you reach a zero balance sooner.
Significant Interest Savings: The combination of reduced principal and a shorter term leads to substantial savings on the total interest paid over the loan's life.
This calculator takes your regular P&I payment and adds your specified extra monthly payment to simulate the accelerated payoff. It then determines the new loan term and total interest saved compared to a standard payment schedule.
When to Consider Extra Payments:
You have a stable income and emergency fund.
Your mortgage has a relatively high interest rate.
You receive windfalls like bonuses or tax refunds.
You want to build equity faster or own your home outright sooner.
Always ensure your extra payments are clearly designated for principal when submitting them to your lender to maximize their benefit.
function calculateMortgage() {
var principal = parseFloat(document.getElementById("loanAmount").value);
var annualRate = parseFloat(document.getElementById("annualInterestRate").value);
var years = parseInt(document.getElementById("loanTermYears").value);
var extraMonthlyPayment = parseFloat(document.getElementById("extraPayment").value);
var resultValueElement = document.getElementById("result-value");
var monthlyPaymentResultElement = document.getElementById("monthlyPaymentResult");
var totalInterestResultElement = document.getElementById("totalInterestResult");
var totalPrincipalResultElement = document.getElementById("totalPrincipalResult");
var numberOfPaymentsResultElement = document.getElementById("numberOfPaymentsResult");
var totalAmountPaidResultElement = document.getElementById("totalAmountPaidResult");
var timeSavedResultElement = document.getElementById("timeSavedResult");
resultValueElement.innerText = "–";
monthlyPaymentResultElement.innerText = "–";
totalInterestResultElement.innerText = "–";
totalPrincipalResultElement.innerText = "–";
numberOfPaymentsResultElement.innerText = "–";
totalAmountPaidResultElement.innerText = "–";
timeSavedResultElement.innerText = "–";
if (isNaN(principal) || isNaN(annualRate) || isNaN(years) || isNaN(extraMonthlyPayment) || principal <= 0 || annualRate < 0 || years 0) {
regularMonthlyPayment = principal * (monthlyRate * Math.pow(1 + monthlyRate, numberOfMonths)) / (Math.pow(1 + monthlyRate, numberOfMonths) – 1);
} else {
regularMonthlyPayment = principal / numberOfMonths;
}
if (isNaN(regularMonthlyPayment) || !isFinite(regularMonthlyPayment)) {
alert("Calculation error for regular monthly payment. Please check inputs.");
return;
}
var totalPaymentWithExtra = regularMonthlyPayment + extraMonthlyPayment;
var remainingBalance = principal;
var totalInterestPaid = 0;
var totalPrincipalPaid = 0;
var monthsPaid = 0;
while (remainingBalance > 0) {
var interestForMonth = remainingBalance * monthlyRate;
var principalForMonth = totalPaymentWithExtra – interestForMonth;
// Ensure we don't overpay the principal in the final payment
if (principalForMonth > remainingBalance) {
principalForMonth = remainingBalance;
totalPaymentWithExtra = interestForMonth + principalForMonth; // Adjust final payment amount
}
remainingBalance -= principalForMonth;
totalInterestPaid += interestForMonth;
totalPrincipalPaid += principalForMonth;
monthsPaid++;
if (monthsPaid > (years * 12 * 5)) { // Safety break for very long/impossible calculations
alert("Calculation taking too long. Please check your inputs.");
return;
}
}
if (isNaN(totalInterestPaid) || !isFinite(totalInterestPaid)) {
alert("Calculation error for total interest. Please check inputs.");
return;
}
var totalAmountPaid = totalPrincipalPaid + totalInterestPaid;
// Calculate for standard payment without extra
var regularTotalInterest = 0;
var regularMonthsPaid = 0;
var tempBalance = principal;
while (tempBalance > 0) {
var interestForMonth = tempBalance * monthlyRate;
var principalPaidThisMonth = regularMonthlyPayment – interestForMonth;
if (principalPaidThisMonth > tempBalance) {
principalPaidThisMonth = tempBalance;
}
tempBalance -= principalPaidThisMonth;
regularTotalInterest += interestForMonth;
regularMonthsPaid++;
if (regularMonthsPaid > (years * 12 * 5)) break; // Safety break
}
var timeSavedYears = Math.floor(((regularMonthsPaid || numberOfMonths) – monthsPaid) / 12);
var timeSavedMonths = Math.floor(((regularMonthsPaid || numberOfMonths) – monthsPaid) % 12);
var timeSavedString = "";
if (timeSavedYears > 0) timeSavedString += timeSavedYears + " year" + (timeSavedYears > 1 ? "s" : "");
if (timeSavedMonths > 0) {
if (timeSavedString.length > 0) timeSavedString += ", ";
timeSavedString += timeSavedMonths + " month" + (timeSavedMonths > 1 ? "s" : "");
}
if (timeSavedString.length === 0 && regularMonthsPaid > monthsPaid) timeSavedString = "Less than a month";
if (timeSavedString.length === 0 && regularMonthsPaid <= monthsPaid) timeSavedString = "No time saved";
resultValueElement.innerText = "$" + parseFloat(totalAmountPaid.toFixed(2)).toLocaleString();
monthlyPaymentResultElement.innerText = "$" + parseFloat(regularMonthlyPayment.toFixed(2)).toLocaleString();
totalInterestResultElement.innerText = "$" + parseFloat(totalInterestPaid.toFixed(2)).toLocaleString();
totalPrincipalResultElement.innerText = "$" + parseFloat(principal.toFixed(2)).toLocaleString(); // Should be equal to loan amount if paid off
numberOfPaymentsResultElement.innerText = monthsPaid.toString();
totalAmountPaidResultElement.innerText = "$" + parseFloat(totalAmountPaid.toFixed(2)).toLocaleString();
timeSavedResultElement.innerText = timeSavedString;
}