Mortgage Overpayment Calculator
Compare regular and lump-sum mortgage overpayments, projected interest savings, term reduction and lender fees in your selected currency.
Compare your current plan with overpayments
| Measure | Without overpayments | With overpayments |
|---|---|---|
| Payoff time | — | — |
| Total interest | — | — |
| Estimated payoff month | — | — |
| Total extra paid | — | — |
| Year | Baseline closing balance | Overpayment closing balance | Overpayment interest | Principal repaid |
|---|
Saved scenarios on this device
How the estimate is calculated
The baseline and overpayment schedules use the same fixed annual rate divided by 12. Each month, interest is calculated from the opening balance. The scheduled payment is applied, followed by any regular overpayment and then a lump sum in its selected month. The final payment is capped at the exact amount due.
- Fixed rate and monthly interest for the whole projection.
- No daily-interest adjustment, recasting, offset balance, rate change or lender overpayment cap.
- Taxes, insurance and other housing costs are excluded.
How the calculation works
This tool keeps the selected currency as a display and input context. It does not convert exchange rates, apply country-specific tax law or claim that a lender will reproduce the estimate exactly. Calculations run locally in the browser.
Read the mortgage overpayment guide, timing assumptions and examples →
Method reviewed 11 July 2026. Formula assumptions, practical examples and primary-source context are published in the linked guide and visible on this page.
Mortgage Overpayment Calculator FAQ
What does the Mortgage Overpayment Calculator model?
It compares a baseline fixed-rate repayment schedule with a schedule containing the regular overpayment and optional lump sum you enter. Interest is applied monthly and the scheduled payment is kept unchanged, so overpayments shorten the projected term.
Does the selected currency change the mathematics?
No. The currency selector changes symbols and number formatting only. The amortisation formula is currency-neutral and does not perform foreign-exchange conversion.
Can I enter my actual mortgage payment?
Yes. Select the actual-payment option when your contractual payment differs from the payment calculated from balance, rate and remaining term. The tool then projects both schedules from that entered payment.
When is a lump-sum overpayment applied?
The lump sum is applied in the selected month after that month’s scheduled payment and any regular overpayment. A lender using daily interest or different timing may produce a different result.
Are early-repayment charges included?
Only if you enter a fee. The tool subtracts that fee from gross projected interest savings to show a fee-adjusted saving, but it cannot determine your lender’s actual allowance or charge.
What if the scheduled payment does not cover the interest?
The calculator stops presenting a payoff date if the balance is not repaid within the simulation limit. Check the payment, interest rate and remaining term against your mortgage statement.
Is this financial advice?
No. It is a planning estimate. Check overpayment limits, product terms, rate changes and charges with your lender or an appropriately regulated adviser before making a significant payment.