Mortgage Overpayment Calculator and Methodology

How Overpayment Reduces Interest

Mortgage interest is charged on the outstanding balance each month. Reducing the balance by any amount, at any time, reduces the interest charged from that point forward. The effect compounds because the reduced interest leaves more of the next monthly payment available to repay capital, which reduces interest further, and so on. For a 25-year loan, this compounding is significant.

There are two ways to apply an overpayment. You can keep the monthly payment the same and shorten the term (the default at most lenders), or keep the term the same and reduce the monthly payment. Shortening the term saves more interest because the overpayment compounds over the full remaining period; reducing the monthly payment improves monthly cash flow but saves less interest. Unless cash flow is tight, shorten the term.

Loan size	Monthly overpayment	Interest saved	Years shaved off
£150,000 @ 4.30%, 25y	£50	£9,100	1y 10m
£150,000 @ 4.30%, 25y	£100	£17,000	3y 5m
£150,000 @ 4.30%, 25y	£200	£29,500	6y 2m
£250,000 @ 4.30%, 25y	£100	£18,700	2y 3m
£250,000 @ 4.30%, 25y	£250	£40,200	5y 4m

The 10% Rule and Early Repayment Charges

Nearly every UK fixed-rate product allows overpayments of up to 10% of the outstanding balance each year without triggering an early repayment charge. Some allow 10% of the original loan rather than the outstanding balance; Nationwide and Santander use outstanding balance, Halifax and Barclays use the original. The difference is small in year one but grows each year. Read the product KFI for the exact wording.

If you overpay more than 10% in a single year you pay the ERC on the excess, not the whole overpayment. On a 3% ERC and a £5,000 excess overpayment, that is £150 — usually still worth paying if the mortgage rate is above 4%, because the interest saved over the remaining fixed period will exceed £150. But run the calculation rather than assume.

The 10% allowance resets on the anniversary of the product start date for most lenders. A £15,000 overpayment in December plus a £15,000 overpayment the following January works fine if the anniversary falls between them; plan the timing.

Worked Example 1: Regular £100 Monthly Overpayment

Lisa has a £180,000 balance, 23 years remaining, on a new 4.30% five-year fixed. Standard monthly payment is £986. She sets up a standing order for a £100 monthly overpayment starting in April 2026.

Without overpayment, total interest over 23 years is £92,127 and the loan clears in April 2049. With the £100 monthly overpayment (assuming rate holds at 4.30%, which is unrealistic but useful for comparison), total interest is £75,300 and the loan clears in October 2046. Saving: £16,827 in interest and 2 years 6 months of payments. Over the five-year fixed period alone, she saves £2,100 of interest.

At remortgage in year five, her balance is roughly £11,800 lower than it would have been, which is also meaningful because it may push her into a lower LTV tier and unlock cheaper rates. For a property worth £280,000, dropping from 62.8% LTV to 58.6% LTV at remortgage moves her from the 65% tier to the 60% tier and typically knocks 0.10% to 0.20% off the rate.

Worked Example 2: Lump Sum vs Regular

David has £12,000 in savings earning 4.1% in a 1-year fixed ISA. His mortgage is £210,000 at 4.50% with 18 years left and standard payment £1,341. Option A: pay £12,000 as a lump sum now. Option B: keep it in the ISA and overpay £200 per month from salary.

Option A saves £8,730 in interest over the remaining term and shortens it by 1 year 4 months. However, the £12,000 no longer earns 4.1% ISA interest — a post-tax equivalent cost of £492 per year of forgone savings growth for as long as the money would have stayed invested. For a 2-year view (before ISA rate renewal uncertainty), that is £984 of forgone savings growth, so net gain of lump sum in the short term is £8,730 − £984 = £7,746.

Option B, £200 per month from salary over five years, saves £4,400 over the fixed period and £23,800 total if continued for the full remaining term. The £12,000 ISA keeps earning, providing emergency liquidity. The right choice depends on emergency fund adequacy. If the £12,000 is David's entire buffer, keep it in the ISA.

Worked Example 3: ISA vs Overpayment Crossover

The break-even for overpayment versus investment is simple on paper: if your mortgage rate is X% and you can earn a post-tax return of Y%, overpay if X > Y, invest if Y > X. At 4.30% mortgage rate, an ISA must beat 4.30% post-tax to win, which cash ISAs currently do not but equity ISAs historically have over ten-year periods.

The complication is risk-adjusted return. Paying down mortgage is a guaranteed, risk-free return of 4.30%. Equity ISA returns are expected value around 5% to 6% real but with material volatility and drawdown risk. For a 25-year horizon, equity ISA usually wins in expected value; for a 5-year horizon with risk aversion, overpayment usually wins because guaranteed returns matter more.

Pension contributions (SIPP or workplace matching) add tax relief of 20% to 45%. For a higher-rate taxpayer, a £80 contribution becomes £100 in the pension, which is an immediate 25% risk-free uplift. Pension almost always beats overpayment for higher-rate taxpayers under 55; the trade-off is illiquidity until age 57 (rising to 58 in 2028).

Offset Mortgages vs Overpayment

An offset mortgage lets savings in a linked account reduce the interest charged on your mortgage without actually paying down the balance. £15,000 in the offset against a £200,000 mortgage means interest is calculated only on £185,000. You keep access to the savings and still save the interest; the effective return on your savings equals the mortgage rate tax-free.

Offset mortgages typically carry rates 0.20% to 0.40% higher than equivalent non-offset products. The offset works for you only if your average linked savings balance over the year is high enough to offset that rate premium. On a £200,000 loan, a 0.30% rate premium is £600 per year; you need roughly £40,000 average offset balance to break even (at 4.30% mortgage rate, £40,000 offset saves £1,720 of interest, minus £600 premium = £1,120 net).

Offset is particularly strong for self-employed borrowers holding VAT and corporation tax reserves, and for households expecting inheritance or bonus lump sums that may come and go. Barclays, Yorkshire and Scottish Widows are the main offset lenders in 2026.

Administrative Points That Trip People Up

When you set up a standing order overpayment, confirm with the lender how the payment is allocated. Most lenders need a note on the payment reference (e.g. "OVERPAYMENT" or your account plus "OVP") or a separate instruction to treat it as overpayment rather than early payment of next month's bill. If misallocated, the payment can be held as a credit rather than reducing the balance, so no interest is saved.

Some lenders recalculate the monthly payment annually rather than continuously; others recalculate only at product change. If yours recalculates annually, your overpayments reduce interest throughout the year but the scheduled payment does not drop until the next review. This is fine for term reduction but confusing if you were expecting the monthly payment to fall.

Keep evidence of overpayments. When you remortgage, a new lender will ask for current balance; if your existing lender's system has not recorded a recent overpayment, you may be shown a higher balance than is correct. Statement printouts and standing order records resolve this quickly.

When Not to Overpay

Five situations where overpayment is the wrong choice. First, if you have any credit at an interest rate higher than your mortgage rate — pay that off first. Credit card debt at 21.9% APR is 5x more expensive than a 4.30% mortgage. Second, if your emergency fund is below three months of essential outgoings; an illiquid overpayment is not a good substitute for liquid savings.

Third, if you are a higher-rate taxpayer under 50 with unused pension annual allowance; pension contributions usually beat overpayments on a risk-adjusted tax-adjusted basis for this cohort. Fourth, if you are close to the 10% annual limit and would trigger ERCs on the excess; time the overpayment into the next product year, or overpay in January and again in the following April if the anniversary is between.

Fifth, if you plan to move house or remortgage at a significantly lower LTV in the next 12 months; the overpayment may push you across an LTV tier boundary too late to benefit, or the lump sum might be better held as deposit contingency. The FOS has upheld complaints where brokers recommended overpayments without considering imminent house moves.