Mortgage Overpayment Calculator
See what overpaying your mortgage could save. Enter your balance, rate and remaining term, then a monthly overpayment and/or a one-off lump sum — the calculator shows the interest saved and how many years and months you'd clear it early, checks it against your 10% annual allowance, and charts the balance with vs without overpaying.
£36,280 interest saved
Paid off: 6y 1m earlier · Monthly payment: £1,112 (unchanged)
Total interest: £97,219 vs £133,499 without overpaying
■ With overpayments · ■ Without (original schedule)
Year-by-year balance
| Year | With overpayment | Without |
|---|---|---|
| 1 | £193,119 | £195,569 |
| 2 | £185,922 | £190,935 |
| 3 | £178,395 | £186,088 |
| 4 | £170,522 | £181,018 |
| 5 | £162,287 | £175,716 |
| 6 | £153,673 | £170,170 |
| 7 | £144,664 | £164,369 |
| 8 | £135,242 | £158,301 |
| 9 | £125,386 | £151,955 |
| 10 | £115,077 | £145,317 |
| 11 | £104,295 | £138,374 |
| 12 | £93,018 | £131,113 |
| 13 | £81,222 | £123,517 |
| 14 | £68,885 | £115,573 |
| 15 | £55,981 | £107,264 |
| 16 | £42,484 | £98,573 |
| 17 | £28,367 | £89,483 |
| 18 | £13,601 | £79,975 |
| 19 | £0 | £70,030 |
| 20 | £0 | £59,629 |
| 21 | £0 | £48,750 |
| 22 | £0 | £37,371 |
| 23 | £0 | £25,469 |
| 24 | £0 | £13,020 |
| 25 | £0 | £0 |
Assumes a repayment mortgage, interest charged monthly, and that overpaying shortens the term (payments unchanged). Figures ignore early repayment charges — most fixed deals let you overpay up to 10% a year fee-free. How we calculate →
How mortgage overpayments work
An overpayment is any money you pay towards your mortgage on top of your normal monthly payment. Because it comes straight off the capital you owe, you're charged interest on a smaller balance every month after — so you save interest and clear the mortgage sooner. This calculator keeps your monthly payment the same and shows how much interest you save and how many years and months you knock off the term, with a chart comparing the balance with and without overpaying.
You can usually choose to shorten the term (keep payments the same — the bigger interest saver) or reduce your monthly payment and keep the term. This tool models the term-reduction route; ask your lender which options they allow.
The 10% annual allowance and early repayment charges
Most fixed-rate UK mortgages let you overpay up to 10% of the outstanding balance each year without penalty (some lenders, such as NatWest, allow 20%; trackers and standard variable rates are often unlimited). Go over your allowance during a fixed deal and you may be hit with an early repayment charge (ERC) — typically 1–5% of the amount repaid. Set your allowance in the calculator and it flags when your planned overpayments would breach it, so you can stay fee-free. Always check your own mortgage offer for the exact figure and reset date.
One-off lump sum vs regular monthly overpayments
Both work, and you can combine them — a lump sum now plus a smaller regular monthly top-up. A lump sum paid early saves the most because the balance is reduced for longer; regular overpayments build the habit and compound over time. Remember both count towards the same annual allowance, so add them together when checking the 10% limit. Enter either or both above to see the combined effect on interest and term.
Should you overpay your mortgage, or save instead?
Compare your mortgage rate to what your money could earn elsewhere after tax. If your mortgage rate is higher than an easy-access savings or ISA rate (after any tax on the interest), overpaying usually wins. If a savings or ISA rate beats it, saving may be better — and keeps the money accessible. Before overpaying, it's sensible to clear higher-interest debt, keep an emergency fund, and make sure you're not sacrificing valuable pension contributions.
Frequently asked questions
How much can I overpay on my mortgage without a penalty?
Most fixed-rate UK mortgages let you overpay up to 10% of the outstanding balance each year fee-free; some lenders (for example NatWest) allow 20%, and many tracker or standard variable rate deals have no limit. Going over your allowance during a fixed period usually triggers an early repayment charge, so check your mortgage offer.
What is an early repayment charge (ERC)?
An ERC is a fee for repaying more than your annual allowance, or leaving a fixed deal early. It's typically 1–5% of the amount repaid and is set out in your mortgage offer. Overpayment calculators, including this one, ignore ERCs in the headline figure, so confirm yours before making a large overpayment.
Does overpaying reduce my term or my monthly payment?
You can usually pick. Keeping payments the same shortens the term and saves the most interest; alternatively some lenders let you lower the monthly payment and keep the original term. This calculator models the term-reduction route, which is generally the bigger saver.
Can I make a one-off lump sum and regular overpayments together?
Yes — many lenders let you combine a one-off lump sum with regular monthly overpayments. Both count towards the same annual allowance (usually 10% of the outstanding balance), so add them together to stay penalty-free. The calculator lets you enter both at once.
Is it better to overpay my mortgage or put the money in savings?
Compare your mortgage rate with the after-tax return on savings. If the mortgage rate is higher, overpaying typically wins; if a savings or ISA rate beats it after tax, saving may be better and keeps the money accessible. Clear higher-interest debt and keep an emergency fund first.
Can you overpay an interest-only mortgage?
Yes. On an interest-only deal an overpayment reduces the capital you'll owe at the end of the term rather than your monthly interest payment. The same annual overpayment allowance and early repayment charge rules apply, so the 10% limit still matters.
Researched & verified by the Calcuris Data & Research Team. How we build and check our tools →