A mortgage is likely the largest loan you'll ever take on, yet few homeowners understand the math behind their monthly payment. In this guide, we'll break down exactly how mortgage calculations work — no finance degree required.
Every mortgage payment is split between two things: principal (the amount you borrowed) and interest (the lender's fee). In the early years, almost all of your payment goes to interest. Over time, the balance shifts toward principal.
The Amortization Formula
Lenders use a standard formula to calculate your fixed monthly payment:
M = P × [r(1+r)^n] / [(1+r)^n − 1]
Where M is the monthly payment, P is the loan principal, r is the monthly interest rate (annual rate divided by 12), and n is the total number of payments (years × 12).
What Drives Your Payment?
Three factors determine your monthly payment: the loan amount, the interest rate, and the term. A lower rate or longer term reduces the monthly cost — but a longer term means you pay far more interest overall.
How to Save Money
- Make biweekly instead of monthly payments to add one extra payment per year.
- Round up your payment — even $50 extra per month can save tens of thousands.
- Refinance when rates drop significantly below your current rate.
- Avoid extending your term when refinancing; keep the payoff timeline short.
Use our mortgage calculator to model different scenarios and see exactly how much interest you could save.