Mortgage tool

Mortgage overpayment calculator

Estimate how a regular monthly overpayment and a one-off lump sum could reduce mortgage interest and shorten the remaining term.

Inputs

Use your current balance, remaining term, interest rate, and the overpayments you are considering. The calculator assumes the rate stays fixed.

Method

It first calculates the standard repayment, then applies the one-off overpayment at the start and adds the monthly overpayment to each payment.

Limits

Check your lender rules before overpaying. This estimate does not include fees, early repayment charges, daily interest, rate changes, payment holidays, or remortgaging.

Last reviewed: 16 June 2026Calculation type: repayment mortgage estimate

How to read the overpayment result

This calculator estimates the interest and time saved when extra payments reduce the mortgage balance faster than the original schedule. It is intended for repayment mortgages where each monthly payment includes interest and capital repayment.

Balance and termUse the current balance and remaining term from your latest mortgage statement or online account.
Interest rateEnter the current annual rate. The calculator assumes this rate stays the same for the full remaining term.
Monthly overpaymentThis is added to the normal calculated repayment every month until the mortgage is repaid.
One-off overpaymentThis is applied at the start of the estimate, reducing the balance before the monthly schedule is calculated.

Worked example

With a £200,000 balance, 25 years remaining, and a 4.75% rate, the standard repayment is about £1,140 per month. Adding a £5,000 one-off payment and £200 per month overpayment reduces the estimated term to about 18 years and 2 months, saving about £46,041 in interest in this simplified model.

Before overpaying

Overpayments can reduce interest, but they are not automatically the best use of spare cash. Check the mortgage conditions before relying on the estimate.

Check your allowanceMany fixed-rate mortgages limit annual overpayments, often as a percentage of the balance. Exceeding the allowance can trigger an early repayment charge.
Check emergency savingsMoney paid into a mortgage may be hard to access again unless the lender allows drawdown or payment holidays.
Compare alternativesHigh-interest debt, pension contributions, savings interest, tax position, and upcoming remortgage plans can all change the decision.
Use official documentsYour mortgage offer, annual statement, and lender account are the best sources for balance, rate, term, and overpayment rules.

Does this work for interest-only mortgages?

No. The calculator is built around repayment mortgage amortisation. Interest-only mortgages need a different model because the normal monthly payment does not reduce the capital balance.

Why does the result assume the rate stays fixed?

Future rates are unknown and remortgage choices vary. A fixed-rate assumption makes the calculation transparent, but it means the result should be treated as an estimate rather than a forecast.

Can overpayments reduce my monthly payment instead of the term?

Some lenders recalculate payments after overpayments, while others keep payments similar and reduce the term. This calculator models the term-reduction style because it shows the interest-saving effect clearly.