Mortgage Calculator

Calculate your monthly mortgage repayment, total interest over the life of the loan, and a full amortisation schedule from your loan amount, interest rate, and term, then re-run it with different rates to see how they would change your payment.

How it works

  1. Enter the home price, your deposit amount, and the loan term in years.
  2. Add the annual interest rate offered by your lender.
  3. Choose your preferred repayment frequency — monthly, fortnightly, or weekly.
  4. The calculator instantly shows your repayment amount, total interest paid, and the full cost of the loan.

The formula behind the repayment

This calculator models a standard repayment (capital-and-interest) mortgage at a fixed rate. The loan amount is the property price minus your deposit. The monthly rate r is the annual rate divided by 12, and n is the number of monthly payments (years x 12). The fixed monthly payment is M = P x r x (1 + r)^n / ((1 + r)^n - 1), where P is the loan amount. At a 0% rate the payment is simply P / n.

Each month the interest due is the outstanding balance multiplied by r. Whatever is left of the payment after interest reduces the balance. Because the balance is highest at the start, early payments are mostly interest and later payments are mostly principal. That is why the equity curve in the Repayment journey tab starts slowly and then steepens, and why overpaying early saves far more than overpaying late.

A worked example

Take a 300,000 property with a 30,000 deposit (10%), so the loan is 270,000, at 4.5% over 25 years. The monthly payment is 1,500.75. Over 300 payments you repay about 450,224 in total, of which about 180,224 is interest. In other words, interest adds roughly two-thirds on top of what you borrowed.

Now add an overpayment of 200 a month in the What-if scenarios tab. The loan is cleared after 242 months instead of 300, which is 58 months (4 years and 10 months) sooner, and total interest falls to about 141,069, a saving of roughly 39,155. The rate stress test shows how sensitive the same loan is to the rate: at 3.5% the payment is 1,351.68, and at 6.5% it is 1,823.06, which is about 322 a month more than at 4.5%.

How to use the three tabs

The Repayment journey tab shows how much of the loan you own over time, when you cross 25%, 50% and 75% equity, and how the total cost splits between principal and interest. The Year by year tab shows each year's principal and interest as bars and a table with the remaining balance. The What-if scenarios tab holds the overpayment simulator and a rate stress test that recalculates the loan at rates from 2 points below to 3 points above the one you entered.

  • Enter the deposit as an amount or as a percentage of the price; the other figure is shown beneath it.
  • Change the currency symbol freely. Nothing is converted, because the maths is the same in any currency.
  • Use the stress test to check whether you could still afford the payment if your rate reset higher when a fixed period ends.

What this calculator leaves out

The figures cover principal and interest only. Real mortgages usually carry extra costs that are not modelled here: arrangement and valuation fees, buildings and life insurance, property taxes, ground rent or service charges, and early-repayment charges if you overpay beyond your lender's allowance. It also assumes one fixed rate for the whole term, whereas many loans fix the rate for two to five years and then move to a variable rate.

Lenders in different countries also compound interest differently. Monthly compounding, as used here, is the common convention, but some markets differ (Canadian fixed-rate mortgages, for example, compound semi-annually), so your lender's own illustration can differ from this result by a small amount. Treat this tool as a way to compare scenarios quickly, and use your lender's official illustration for the final numbers.

Frequently asked questions

How is a monthly mortgage payment calculated?

It uses the standard amortisation formula — a fixed monthly payment worked out from the loan amount, the monthly interest rate, and the number of payments, so the loan is fully paid off in equal instalments over the term.

Why does my total interest paid look so much higher than the loan amount?

Interest compounds on the remaining balance every month over a 25 to 30 year term — on a typical mortgage the total interest paid over the full term can end up close to, or even exceed, the original loan amount, especially at higher rates.

Sources and further reading

Formulas checked against the sources below. Last reviewed . General information only, not professional advice.