UK Mortgage Overpayment Calculator
See how a regular extra payment or a one-off lump sum shortens an existing UK repayment mortgage. This is not a new-purchase quote and not a repayment-versus-interest-only comparison — those jobs live on the UK repayment calculator. Stamp Duty is a separate purchase cost on the UK stamp duty calculator.
Why this is its own page
The user job here is “I already have a repayment mortgage — what does an extra £X a month, or a lump sum, do?” That is not the same job as choosing repayment versus interest-only on a purchase, not the same job as a US PITI quote, and not the same job as Stamp Duty on the purchase. Publishing those as one URL would hide the number that actually changes.
Formula
The contractual monthly payment is standard amortisation on the outstanding balance:
P = outstanding balance · i = monthly rate · n = months remaining
Each month, interest is balance × i. The rest of the payment (scheduled + extra) reduces the balance. A lump sum is subtracted from the opening balance before the first month. We count months until the balance hits zero, then compare interest paid with and without the extras. Early-repayment charges are not deducted from interest saved; the 10% allowance check is a separate warning, because the actual cap is a product rule, not part of amortisation.
By Suhaib Hassan · Formula verified 20 Sep 2026 · Method v1.1-UK-overpay · How CalculatePilot verifies formulas
A worked sterling example (pre-computed)
Outstanding balance £200,000 · rate 4.5% · 20 years remaining (240 months) · extra £200 each month. Figures use the same amortisation as the calculator, so the answer is readable if JavaScript is off.
No overpayment
Scheduled payment ≈ £1,265.30
Months remaining = 240 (20 years)
Interest ≈ £103,672
Plus £200 a month
New payment ≈ £1,465.30
Months remaining = 192 (16 years)
Interest ≈ £80,735
Time saved = 48 months (4 years)
Interest saved ≈ £22,936
The extra £200 does not just knock four years off the calendar. It also avoids four years of interest on a shrinking balance. That is the unique output of this page.
What this figure does not include
- Early repayment charges. Many fixed deals cap penalty-free overpayments (often around 10% of the outstanding balance per year). The readout compares year-1 extras with that typical cap. Breach the actual cap in your offer and the ERC can erase the interest you modelled.
- Stamp Duty. A purchase-time tax, not an overpayment. Use the UK Stamp Duty Calculator.
- Offset accounts. Savings held against the loan reduce interest a different way. This page models a cash extra payment, not an offset balance.
- Rate switches. A remortgage to a lower rate is a different job — use the refinance breakeven calculator.
- A new purchase. For repayment versus interest-only on a sterling loan, use the UK repayment calculator.
MoneyHelper explains overpayments and ERC limits in plain language — start with their overpayment guidance before you instruct the lender.
Frequently asked questions
Is this the same as the UK repayment vs interest-only calculator?
No. That page compares two products. This page takes an existing repayment mortgage and measures the effect of a regular extra payment.
Do UK lenders allow overpayments?
Most repayment mortgages allow them, often up to a yearly percentage of the balance without an ERC. Check your offer document for the cap and whether extra payments cut the term or the instalment.
Does this include early repayment charges?
Interest saved is amortisation only. The 10% year-1 check is a typical penalty-free allowance, not your lender’s ERC table.
Should I overpay or remortgage?
Overpaying keeps the current rate and shortens the term. Remortgaging changes the rate and usually adds fees. Use this page for extra payments; use refinance breakeven if the question is a new rate.
Are these figures a lender illustration?
No. Confirm with your lender or a regulated adviser, especially around ERC limits.
Next calculation
Estimate only. UK overpayment · method v1.1-UK-overpay · last verified 20 Sep 2026 · sources and verification · who builds CalculatePilot