Mortgage Calculator: How Your Monthly Home Loan Payment Is Worked Out
A mortgage calculator turns a home price, deposit, interest rate and term into one number you can actually plan around: the monthly payment. Enter the loan amount, the annual rate and the number of years, and you instantly see the repayment, the total interest you will pay over the life of the loan, and how much of your first payment goes to interest rather than principal.
What the mortgage payment formula does
Almost every fixed-rate mortgage uses an amortising payment: the same amount is paid every month, but the split between interest and principal changes. Early on, most of the payment covers interest on a large outstanding balance. As the balance falls, more of each payment attacks the principal, which is why the last years of a mortgage clear the debt so much faster than the first.
The monthly rate is the annual rate divided by twelve, and the number of payments is the term in years multiplied by twelve. Those two inputs, plus the amount borrowed, are all the maths needs.
Which inputs move the payment most
- Interest rate: a one-point rate change on a 30-year loan typically moves the payment by roughly 10-12%.
- Term: a 15-year term raises the monthly payment but can halve the total interest compared with 30 years.
- Deposit: a larger down payment reduces both the balance and, often, the rate you are offered.
- Extra payments: any amount above the scheduled payment goes straight to principal and shortens the term.
What this calculator does not include
The result is principal and interest only. Property tax, buildings insurance, mortgage insurance, HOA or service charges and any lender fees are added on top by your lender, and they can raise the real monthly outgoing by a significant margin. Treat the figure here as the loan cost, then budget for the rest separately before you decide what you can afford.
How to use the result when comparing offers
Run each lender's rate and term through the calculator and compare total interest, not just the monthly payment. A lower payment stretched over a longer term almost always costs more overall. If two offers are close, test what happens when you add a small overpayment each month — it is usually the cheapest way to cut years off the loan.
Key takeaways — Mortgage Calculator
- Monthly payment depends on three things: amount borrowed, monthly interest rate and number of payments.
- Compare total interest across offers, not just the headline monthly figure.
- Taxes, insurance and fees are extra — add them before judging affordability.