Mortgage Overpayment Calculator
Work out how much interest you save and how much sooner you are mortgage-free when you overpay monthly or as a lump sum on a repayment mortgage.
Interest saved
£18,020
Mortgage-free sooner by
3 years and 5 months
New remaining term
21 years and 7 months
Current monthly payment
£1,055.67
Interest without overpaying
£116,702
Interest with overpaying
£98,682
Remaining balance by year
- Without overpaying
- With overpaying
Show the table
| Year | Without overpaying | With overpaying |
|---|---|---|
| 0 | £200,000 | £200,000 |
| 1 | £195,245 | £194,023 |
| 2 | £190,297 | £187,803 |
| 3 | £185,147 | £181,329 |
| 4 | £179,787 | £174,591 |
| 5 | £174,209 | £167,579 |
| 6 | £168,404 | £160,282 |
| 7 | £162,362 | £152,687 |
| 8 | £156,074 | £144,782 |
| 9 | £149,530 | £136,556 |
| 10 | £142,719 | £127,994 |
| 11 | £135,630 | £119,083 |
| 12 | £128,253 | £109,810 |
| 13 | £120,576 | £100,158 |
| 14 | £112,585 | £90,114 |
| 15 | £104,269 | £79,660 |
| 16 | £95,614 | £68,780 |
| 17 | £86,607 | £57,457 |
| 18 | £77,232 | £45,673 |
| 19 | £67,476 | £33,409 |
| 20 | £57,322 | £20,645 |
| 21 | £46,755 | £7,361 |
| 22 | £35,756 | £0 |
| 23 | £24,310 | £0 |
| 24 | £12,398 | £0 |
| 25 | £0 | £0 |
Please note: This is an example calculation, not financial advice. Personal circumstances, changes in rules and rates, and additional costs are not included. If in doubt, talk to a financial adviser.
Early repayment charges, tax effects and rate changes during the term are not included.
How this number is calculated
The blocks use your input. Change a value above and they update with it.
Step 1 · Current monthly payment
Step 2 · Balance after the lump sum
Step 3 · Payment plus extra
Step 4 · Months until paid off (exact)
Step 5 · Number of payments
Step 6 · Interest without overpaying
Step 7 · Interest with overpaying
Step 8 · Interest saved
How the calculation works
See what overpaying your mortgage does: how much interest you save and how much sooner you are mortgage-free. Enter your remaining balance, rate and term, and an extra amount per month, as a lump sum, or both.
The calculator assumes a repayment (annuity) mortgage: the same payment every month, made up of interest and principal. When you overpay, the balance goes down and you no longer pay interest on that part. If your monthly payment stays the same, more of it goes to principal from then on and you finish sooner. The calculator works out how many months it takes with the overpayment and compares the interest with and without. The last payment is usually part of a monthly payment; it counts as a whole month.
Formula
Monthly rate r = rate ÷ 100 ÷ 12, months n = remaining term × 12. Payment M = balance × r ÷ (1 − (1 + r)^−n). With overpaying: balance B = balance − lump sum, payment P = M + extra per month, months until paid off = −ln(1 − r × B ÷ P) ÷ ln(1 + r). Interest = payments − principal repaid; saved = interest without − interest with.
Assumptions
- The mortgage is a repayment (annuity) mortgage with a fixed rate for the whole remaining term.
- Your monthly payment stays the same, so overpaying makes you mortgage-free sooner. Some lenders lower the monthly payment instead and keep the term; the saving is then smaller.
- Early repayment charges above the penalty-free allowance are not included.
- Tax effects are not included.
- Amounts are rounded to whole units, the monthly payment to the penny.
Example calculations
The button fills in the calculator above, so you can carry on from there.
-
100 extra per month
Interest saved
£18,620
Remaining balance £250,000.00 · Annual interest rate (%) 4 · Remaining term (years) 25 · Extra per month £100.00 · Lump sum £0.00
-
Lump sum of 10,000
Interest saved
£16,292
Remaining balance £250,000.00 · Annual interest rate (%) 4 · Remaining term (years) 25 · Extra per month £0.00 · Lump sum £10,000.00
-
Monthly and lump sum
Interest saved
£42,359
Remaining balance £300,000.00 · Annual interest rate (%) 3.5 · Remaining term (years) 28 · Extra per month £200.00 · Lump sum £5,000.00
Frequently asked questions
How much do I save by overpaying?
On a remaining balance of 250,000 at 4% with 25 years left, the monthly payment is 1,319.59. Paying 100 extra every month saves 18,620 in interest and makes you mortgage-free 2 years and 10 months sooner.
Is a lump sum or a monthly overpayment better?
A lump sum now works for the longest time. On the same mortgage, a lump sum of 10,000 saves 16,292 in interest and you finish 1 year and 7 months sooner. Paying 100 extra a month adds up to more money over the full term, but it builds up gradually.
Will my payment go down or will I finish sooner?
That depends on your lender. Some reduce the monthly payment after an overpayment and keep the term; others shorten the term, or let you choose. This calculator shows what happens when the payment stays the same.
How much can I overpay without a charge?
In the UK, many fixed-rate deals allow overpayments of up to 10% of the balance per year without an early repayment charge. In the US, prepayment penalties are uncommon on most home loans. Check the terms of your mortgage.
Should I overpay or save?
It depends on your situation. Compare the mortgage rate with what your savings or investments earn after tax, and keep an emergency fund before you overpay.
How exact is the result?
The interest is within one unit of working through the schedule month by month. Lenders may charge interest per day or apply overpayments on a set date, so their figures can differ slightly.
Sources and checks
Sources
- UK House Price Index for July 2026 | GOV.UK (opens in a new tab)
- Money and Credit - August 2026 | Bank of England (opens in a new tab)
- Mortgage repayments explained | Which? (opens in a new tab)
How we check it
This calculator has 11 fixed checks: 6 calculations with a result that is fixed in advance, and 5 cases of invalid input that must be rejected. They all run automatically every time we publish; if one fails, the calculator does not go live.
See the calculations
- Remaining balance £250,000.00 · Annual interest rate (%) 4 · Remaining term (years) 25 · Extra per month £100.00 · Lump sum £0.00 → Interest saved £18,620
- Remaining balance £250,000.00 · Annual interest rate (%) 4 · Remaining term (years) 25 · Extra per month £0.00 · Lump sum £10,000.00 → Interest saved £16,292
- Remaining balance £300,000.00 · Annual interest rate (%) 3.5 · Remaining term (years) 28 · Extra per month £200.00 · Lump sum £5,000.00 → Interest saved £42,359
- Remaining balance £80,000.00 · Annual interest rate (%) 5 · Remaining term (years) 10 · Extra per month £250.00 · Lump sum £0.00 → Interest saved £6,291
- Remaining balance £120,000.00 · Annual interest rate (%) 0 · Remaining term (years) 10 · Extra per month £500.00 · Lump sum £0.00 → Interest saved £0
- Remaining balance £12,000.00 · Annual interest rate (%) 6 · Remaining term (years) 1 · Extra per month £0.00 · Lump sum £1,000.00 → Interest saved £62
Changes
- : Separate default balance and interest rate for the UK, with official UK sources.
- : First version. Sources added and content reviewed.
Last reviewed: · How we build and check our calculators
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