Mortgage overpayment calculator
A hundred a month, years off the end.
An overpayment does two things at once: it kills the interest you would have paid on that money for the rest of the term, and it brings the last payment forward. See both, then see whether saving the money instead would have done better.
Your mortgage
The balance on your latest statement, not what you originally borrowed.
Starts at the market average five-year fix, 5.61% (24 July 2026). Yours is on your mortgage statement.
On top of your normal payment of £1,117 a month.
Paying £100 a month extra saves you
On £180,000 at 5.61% with 25 years to run, the normal payment is £1,117 a month and the interest over the full term is £155,164. Paying £100 a month extra saves £28,390 of interest and clears the mortgage 3 years 11 months earlier.
and hands you the keys 3 years 11 months earlier.
£155,164
£126,774
21 years 1 month
Every pound you overpay stops earning the lender 5.61% a year for the rest of the term. That is the whole trick - there is nothing clever going on.
The picture
Interest handed to the lender, adding up year by year. The overpaid line stops climbing the moment the mortgage is gone - the shaded gap is what you kept.
| Year | Cumulative interest with the overpayment | Cumulative interest with no overpayment |
|---|---|---|
| 0 | £0 | £0 |
| 1 | £9,980 | £10,012 |
| 2 | £19,694 | £19,828 |
| 3 | £29,126 | £29,437 |
| 4 | £38,261 | £38,828 |
| 5 | £47,080 | £47,988 |
| 6 | £55,567 | £56,903 |
| 7 | £63,701 | £65,560 |
| 8 | £71,462 | £73,943 |
| 9 | £78,829 | £82,037 |
| 10 | £85,780 | £89,826 |
| 11 | £92,290 | £97,290 |
| 12 | £98,334 | £104,413 |
| 13 | £103,885 | £111,175 |
| 14 | £108,914 | £117,553 |
| 15 | £113,392 | £123,527 |
| 16 | £117,288 | £129,073 |
| 17 | £120,566 | £134,167 |
| 18 | £123,193 | £138,782 |
| 19 | £125,130 | £142,891 |
| 20 | £126,337 | £146,465 |
| 21 | £126,774 | £149,473 |
| 22 | £126,774 | £151,882 |
| 23 | £126,774 | £153,659 |
| 24 | £126,774 | £154,765 |
| 25 | £126,774 | £155,164 |
Or save it instead?
Overpaying wins, and here is by how much.
£28,390
£23,003
£10,467
Both routes hand over the same money every month for the same 25 years, and both end with the mortgage gone. Overpay and you clear it early, then put the whole payment into savings for the years you have left. Save instead and the money compounds from day one while the mortgage runs its full term. The figures above compare where you land at the end of both.
The rate that makes it a dead heat is 5.61% - your mortgage rate. Overpaying is a guaranteed, tax-free return at exactly what you are being charged, which is why beating it in a savings account is harder than it sounds.
Savings interest here is assumed tax free, as it would be in a cash ISA. Outside one, basic-rate tax takes 20% of anything over your Personal Savings Allowance, which pushes the break-even rate higher still. The best easy-access rate we have on file is 4.21% (28 July 2026).
Before you set one up
- Check your overpayment allowance first. Most fixed and discount deals let you overpay 10% of the outstanding balance a year without penalty. Go past it and the early repayment charge can wipe out everything you just saved.
- Ask them to shorten the term, not cut the payment. Some lenders quietly reduce your monthly payment instead of your term when you overpay. That hands the benefit straight back to them. Say which one you want, in writing.
- Clear the expensive debt first. A credit card at 24% or an overdraft at 39% costs multiples of a mortgage. Overpaying the cheapest debt you own while an expensive one runs is the wrong order.
- Keep an emergency fund back. Money paid into a mortgage is hard to get out again. Three to six months of essential spending somewhere you can reach it comes before any of this.
- Your rate will not stay the same. This assumes today's rate for the whole remaining term. When your fix ends the sums change, so it is worth running this again each time you remortgage.
How this works
Interest is charged monthly on what you still owe, and your normal payment is the standard one that clears £180,000 over 25 years at 5.61%. Overpaying shortens the term rather than cutting the payment. Plenty of lenders work interest out daily instead of monthly, which makes a real overpayment very slightly better than the figures here.
The rate is assumed to hold for the whole remaining term, which it will not - run this again whenever you remortgage. Early repayment charges are not modelled. This is not financial advice. These are ideas to make you a little bit richer.
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