Mortgage Overpayment Calculator UK
Every extra pound earns your mortgage rate — tax-free, guaranteed.
See exactly what overpaying saves: interest avoided, years cut from the term, and your new mortgage-free date — with the typical 10% penalty-free allowance and early repayment charges built into the check. Exact month-by-month maths, not approximations.
Examples use £200,000 at 4.5% over 25 years. Your exact figures appear in the calculator above.
Why small overpayments move mountains
Interest stops on every pound, forever
Each overpaid pound reduces the balance that interest is charged on every month for the rest of the term — the effect compounds. £200 a month on a £200,000 / 4.5% / 25-year mortgage (payment £1,112) saves about £36,000 and brings mortgage freedom forward six years.
Exact, not estimated
This calculator runs the amortisation month by month — the same arithmetic your lender uses — rather than a formula approximation, so the payoff date and interest figures match real statements.
How much you can overpay penalty-free
During a fixed or discounted period, most lenders allow overpayments up to 10% of the outstanding balance per deal year without charge (a few use 10% of the original loan — check your offer document). Trackers and standard variable rates are usually unlimited.
Beyond the allowance, an early repayment charge applies — typically stepping down through the fix, e.g. 5%/4%/3%/2%/1% across a five-year deal. Facing a big lump? Splitting it across two deal years, or waiting for the fix to end, often avoids the charge entirely.
The after-tax comparison that decides it
Overpaying "earns" your mortgage rate tax-free and risk-free. Savings interest above the personal savings allowance is taxed — a 4.5% account is worth ~3.6% to a basic-rate taxpayer and ~2.7% at higher rate. Unless your savings or ISA rate clearly beats the mortgage rate after tax, the mortgage wins.
Two caveats: build a 3–6 month emergency fund first (overpaid money is hard to get back), and higher-rate earners should compare against pension contributions, whose 40%+ relief usually beats both.
Term reduction, payment reduction, and offsets
Tell your lender to shorten the term, not reduce the payment — that's where the interest saving lives, and it's what this calculator models. Keeping the payment level while the balance shrinks is also more flexible than formally shortening the contractual term, because you can stop any time.
If your product offers an offset account, parking cash there gives the identical interest saving with instant access — ideal for money you might need back. Overpay outright only with money you're sure you won't.
❓ Frequently asked Frequently asked questions
How much does overpaying my mortgage save?
On £200,000 at 4.5% over 25 years, £200 a month saves about £36,000 of interest and clears the loan six years early. Every pound overpaid earns your mortgage rate, tax-free.
How much can I overpay without penalty?
Typically 10% of the outstanding balance per year on fixed deals; trackers and SVRs are usually unlimited. Beyond the allowance, ERCs of 1–5% apply during the fix.
Lump sum or monthly overpayments?
Earlier money saves more — a £10,000 lump today saves ~£19,000 on the example mortgage, beating the same total drip-fed over years.
Should I overpay or save instead?
Compare after-tax returns: the mortgage rate is tax-free; savings above the PSA are taxed. Keep an emergency fund first, and higher-rate earners should weigh pension contributions too.
Term reduction or payment reduction?
Term reduction (keeping your payment) maximises interest saved — tell your lender explicitly, as defaults vary.
Do overpayments change my monthly payment?
Not unless you ask — the payment stays the same and the balance falls faster. At remortgage the smaller balance means better loan-to-value pricing too.
Where these figures come from
Amortisation is standard loan mathematics; allowance and ERC conventions reflect UK lender terms.
- Overpayment guidance — MoneyHelper — Paying off your mortgage early.
- Early repayment charges — MoneyHelper — Mortgage fees and costs.
- Personal savings allowance (for the comparison) — GOV.UK — Tax on savings interest.
Last checked: July 2026. Your own allowance and ERC schedule are in your mortgage offer document — always check before large overpayments.
Select the question that matches where you are right now.
Your result is the guaranteed, tax-free return on money sent to the mortgage — interest that never gets charged, and a payoff date pulled years closer.
Compare the interest saved against what the same money earns elsewhere after tax. If the mortgage wins and the emergency fund is full, set up a standing order.
Not accessible money — overpayments are hard to get back without remortgaging. It's also not advice; big lump decisions deserve a broker conversation.
Month-by-month simulation at your exact rate and term. All calculations run in your browser.
Three levers set the size of the win: rate, time, and timing.
At 5.5% the same overpayment saves half again more than at 3.5% — the case for overpaying strengthens with every rate rise.
Early-term overpayments hit hardest — interest compounds on the avoided balance for decades. The same £10,000 saves far less in year 20 than year 2.
Stay within the 10% allowance and the return is pure; breach it in a fix and the 1–5% ERC can erase a year of savings. The calculator flags the boundary automatically.
Small mechanics compound the benefit.
Rounding £1,112 to £1,200 is an invisible £88 overpayment that saves five figures over the term.
If you remortgage cheaper, keep paying the old amount — the difference is a free overpayment your budget already absorbed.
Bonuses and tax refunds sent straight to the mortgage never get absorbed into lifestyle.
Overpaying sits in a family of housing-money decisions.