Mortgage overpayment calculator

How it works

The standard monthly payment is calculated from your balance, rate and term, then two payoff schedules are simulated month by month: one with just that payment, one with your overpayment added on top. Each month's interest is the current balance times the monthly rate, and the rest of the payment reduces the balance — the difference in how many months each schedule takes, and how much total interest each racks up, is what's shown above.

FAQ

Does overpaying always save interest, no matter how my lender applies it?

This calculator assumes every overpayment reduces the outstanding balance immediately, which is how most UK lenders apply regular monthly overpayments. Some lenders only recalculate interest annually rather than monthly — check your mortgage terms, since that changes the exact savings slightly.

Is there a limit to how much I can overpay without a penalty?

Many UK fixed-rate deals cap penalty-free overpayments at 10% of the outstanding balance per year — this calculator doesn't enforce that cap, so check your own lender's allowance before committing to a monthly figure.