Mortgage Calculator
What will it really cost?
A repayment mortgage is mostly interest at the start. See your real monthly payment, what the loan costs in total, and how much a little extra each month takes off the top.
How this is worked out
Your monthly payment is the fixed amount that clears the loan over its term, mostly interest at first, mostly principal near the end. We amortise it month by month, folding in any extra payment, which goes straight to principal and shortens the loan.
This is a repayment (annuity) mortgage at a fixed rate, and it excludes fees, insurance and property tax, so treat it as a guide. Not financial or tax advice.
How a repayment mortgage works
A repayment (or annuity) mortgage is built around one fixed monthly payment that clears the whole loan by the end of its term. Each payment is split two ways: part covers the interest charged on the balance that month, and whatever's left chips away at the principal you actually owe. Early on the balance is large, so most of the payment is interest and only a sliver reduces the debt. As the balance falls the interest shrinks and more of each payment turns into principal, which is why the loan clears slowly at first and quickly at the end.
Why overpaying saves so much interest
Every extra pound, krone or dollar you pay beyond the required amount skips the queue and comes straight off the principal. Because interest is charged on the remaining balance, a smaller balance means less interest every single month for the rest of the loan, and that saving compounds over decades. A modest, consistent overpayment can knock years off the term and save a surprising share of the total interest. Nudge the extra-payment field above and watch the total interest and the payoff date move.
The fixed-rate caveat
This calculator assumes your interest rate stays fixed for the entire term. In reality many mortgages are variable, or fix for a few years and then reset, so a real payment can rise or fall when the rate changes. It also leaves out arrangement fees, buildings insurance and property tax, which sit on top of the figures here. Use it to understand the shape of a loan and the power of overpaying, then confirm the exact numbers with your lender. For how a mortgage fits your wider path to financial independence, see the FIRE calculator, browse all calculators, or read more about Runway.
Overpay the mortgage, or invest instead?
It's the big question: throw spare cash at the loan, or invest it toward financial independence? Runway models both against your real numbers, your country's tax and pension rules, and shows which reaches your freedom age sooner. It launches on the App Store on 25 August 2026, and the beta is open now.
It's the big question: throw spare cash at the loan, or invest it toward financial independence? Runway models both against your real numbers, your country's tax and pension rules, and shows which reaches your freedom age sooner. Your freedom age is free, forever.
Try the beta on TestFlight Download free on the App StoreFrequently asked
How is the monthly payment calculated?
It comes from the standard amortisation (annuity) formula: the loan times the monthly interest rate, scaled so a single fixed payment clears the balance to zero over the term. Each payment covers that month's interest first and the rest cuts the principal, so the split moves from mostly interest to mostly principal as the balance falls.
How much do extra payments save?
It depends on the overpayment and how long is left, but the effect is powerful. Every extra amount goes straight to principal, so you're charged less interest every month after. A steady monthly overpayment can take years off the term and a real slice off the total interest. The tool shows both the years and the interest saved as you change the extra field.
Does this include taxes and insurance?
No. The figures cover only the mortgage itself, the principal and the interest on it. Buildings and contents insurance, property or council tax, arrangement and valuation fees, and any mortgage insurance all sit on top and vary by country and lender, so budget for them separately.
What if my interest rate changes?
This tool assumes one fixed rate for the whole term. Many mortgages are variable, or fix for a few years and then reset, so a real payment can rise or fall over time. If your rate is likely to move, run the calculator at a few different rates to see the range of payments you might face.
Built by Dylan, maker of Runway
Italian, in Norway about eight years, building the cross-border FIRE planner that didn't exist for someone like me. This calculator is an educational tool, not financial or tax advice.
Related: FIRE calculator, Coast FIRE calculator, and the 4% rule.