Estimate mortgage overpayment savings, interest reduction, payoff time, and monthly repayment scenarios.
An overpayment goes straight against the balance, so the interest you would have paid on that amount for the rest of the term never accrues at all. That is why the same overpayment saves far more early in a mortgage than near the end, and why the saving depends on the rate and the years remaining rather than on the size of the payment alone. Lenders usually cap penalty-free overpayments during a fixed deal, so check the limit before making a large one.
The balance remaining, the rate, the years left — and the overpayment you are weighing.
Repaying £1,001 a month — £180,000 at 4.5%, the rate applied monthly as a twelfth of the annual figure, the way UK mortgage lenders quote it — clears it in 300 months. A longer term lowers the monthly payment and raises the total.
Paying £100 a month on top — with the monthly payment held level and the term shortened — clears it after 254 months instead of 300, and saves £20,758 in interest. Check the agreement first: some allow overpayment without charge and some do not.
This is arithmetic on the figures you enter, not debt advice. It assumes the rate and the payment both stay the same and that nothing further is borrowed, which is rarely how a real balance behaves. For free, regulated help, talk to MoneyHelper or StepChange.
| Tax/source year | 2026/27 |
|---|---|
| Region | England/Wales/Northern Ireland |
| Tax code basis | 1257L where the page uses PAYE defaults |
| Pension basis | No pension deduction unless this example says otherwise |
| Student loan basis | No student loan unless selected in this example |
| What is not decided here | PayBreakdown does not decide payroll correctness, lender approval, benefit entitlement or employer compliance, and does not give regulated financial, mortgage, debt, tax, payroll or legal advice. |
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After an overpayment most lenders let you choose what it does: keep the monthly payment the same and finish earlier, or keep the term and pay less each month. The arithmetic is not symmetric. Keeping the payment up means every month from now on clears more principal than the schedule required, so the balance falls faster for the entire remaining life of the loan — the overpayment keeps working. Cutting the payment instead returns the schedule to its original pace and takes the gain as immediate monthly relief, which saves the least interest overall. Neither is wrong: term reduction maximises the saving, payment reduction maximises slack. The calculator above models the first — the payment held level, the term shortened — and says so beside its result, so the biggest available saving is the number on the table when you weigh the trade.
Three checks come before the arithmetic.
Interest rate, term, fees, deposit and the credit commitments you already have all move the result, and a lender applies its own affordability policy and credit checks on top. Treat the number as a way to see whether a repayment looks comfortable or stretched, not as an offer — and read it against your real bills, savings and any income change you expect. See how each deduction is worked out.
No. It is an estimate worked from published HMRC and GOV.UK figures for the current tax year, using the assumptions shown on this page. It cannot see your tax code history, benefits in kind or employer payroll rules, so check important decisions against your payslip, official guidance or a qualified adviser.
Yes. Open the linked calculator to change the salary, pension, tax code, region or student loan plan and the figures update as you type.
Last updated 2026-08-28. Checked by Sean Elsmore. Rates last checked against GOV.UK on 2026-08-07. Estimates are for planning and should be checked against official records where the decision matters.