Mortgage Overpayment Calculator
See how much interest and time you could save by overpaying a repayment mortgage each month or with a one-off lump sum, and check whether an early repayment charge could apply.
Checked 20 October 2026.
Paid every month from the first payment. Monthly overpayments shorten the term.
Interest saved
£41,842.60
Time saved: 6 years 2 months.
- Current monthly payment
- £1,169.18
- New monthly payment
- £1,169.18
- Total interest without overpaying
- £150,754.02
- Total interest with overpaying
- £108,911.43
- Payments left without overpaying
- 300
- Payments left with overpaying
- 226
This is an estimate. Your lender's figures will differ if they calculate interest daily or yearly.
Year by year balance
| Year | Without overpaying | With overpaying |
|---|---|---|
| 1 | £195,876.19 | £193,420.42 |
| 2 | £191,541.39 | £186,504.21 |
| 3 | £186,984.82 | £179,234.15 |
| 4 | £182,195.12 | £171,592.15 |
| 5 | £177,160.38 | £163,559.16 |
| 6 | £171,868.05 | £155,115.19 |
| 7 | £166,304.95 | £146,239.22 |
| 8 | £160,457.23 | £136,909.13 |
| 9 | £154,310.33 | £127,101.70 |
| 10 | £147,848.95 | £116,792.50 |
| 11 | £141,056.99 | £105,955.86 |
| 12 | £133,917.54 | £94,564.79 |
| 13 | £126,412.82 | £82,590.94 |
| 14 | £118,524.15 | £70,004.49 |
| 15 | £110,231.88 | £56,774.09 |
| 16 | £101,515.36 | £42,866.79 |
| 17 | £92,352.88 | £28,247.98 |
| 18 | £82,721.64 | £12,881.23 |
| 19 | £72,597.64 | £0.00 |
| 20 | £61,955.68 | £0.00 |
| 21 | £50,769.26 | £0.00 |
| 22 | £39,010.51 | £0.00 |
| 23 | £26,650.17 | £0.00 |
| 24 | £13,657.45 | £0.00 |
| 25 | £0.00 | £0.00 |
Show the working
- Monthly rate = yearly rate ÷ 12 ÷ 100.
- Monthly payment = balance × monthly rate ÷ (1 − (1 + monthly rate) to the power of minus the number of months) = £1,169.18. At 0% it is the balance ÷ months.
- Interest without overpaying = payment × months − balance = £150,754.02.
- Each month: interest = balance × monthly rate. You pay the monthly payment plus your overpayment (or what is left). The balance goes down by the payment minus the interest.
- Interest saved = £150,754.02 − £108,911.43 = £41,842.60. Time saved = 6 years 2 months.
Nothing you enter leaves your device. Lenders set their own overpayment limits and charges. Check your mortgage offer or ask your lender before overpaying.
Is it worth overpaying my mortgage?
Overpaying cuts the balance, so you pay less interest and can clear the mortgage sooner. MoneyHelper says to check your deal first, because charges can cut into the savings, and to clear more expensive debts before reducing your mortgage.
Compare the saving with what your money could earn elsewhere. MoneyHelper says that if you can find a better rate of return than the rate you are being charged, saving may make more sense. The ISA allowance calculator can help you compare.
What monthly overpayments save
| Overpayment | Time saved | Interest saved |
|---|---|---|
| £50 a month | 1 year 11 months | £13,414.52 |
| £100 a month | 3 years 6 months | £24,505.30 |
| £200 a month | 6 years 2 months | £41,842.60 |
| £300 a month | 8 years 2 months | £54,832.52 |
| £500 a month | 11 years 1 month | £73,102.18 |
Example only, using £200,000 at 5% over 25 years. Interest worked out monthly.
What a one-off lump sum at the start saves (shorter term)
| Lump sum | Time saved | Interest saved |
|---|---|---|
| £5,000 | 1 year 2 months | £11,922.46 |
| £10,000 | 2 years 4 months | £22,884.92 |
| £20,000 | 4 years 5 months | £42,323.45 |
Example only, using £200,000 at 5% over 25 years. Interest worked out monthly.
How much can I overpay without a penalty?
Many lenders let you overpay up to 10% a year without penalties, according to MoneyHelper. Your limit depends on your lender and deal, so check your mortgage terms.
For example, Santander says you can overpay up to 10% of a fixed-rate loan each calendar year without an early repayment charge, and that unused allowance cannot be carried over.
Should I shorten the term or lower the monthly payment?
Shortening the term saves more interest. In the example above, a £10,000 lump sum saves £22,884.92 of interest by shortening the term, but £7,537.70 if it lowers the payment by £58.46 a month.
Santander says regular overpayments reduce the term, while a single overpayment can reduce either. Other lenders' options vary.
What does the FCA say about early repayment charges?
FCA rule MCOB 12.3.1R says an early repayment charge on a regulated mortgage contract must be expressed as a cash value and be a reasonable pre-estimate of the lender's costs.
MCOB 12.3.4R requires the firm to disclose the maximum charge before you take the mortgage, a further advance or switch rates, if a charge applies. The rules do not set an allowance, so check your own terms.
Common questions
Does overpaying reduce my monthly payment?
Not usually. Regular overpayments normally shorten the term and keep the payment the same. Some lenders let you choose to lower the payment after a one-off overpayment, so check with yours.
Do overpayments count against my yearly allowance?
Usually yes. Any extra money you pay in the allowance period counts, including lump sums and regular overpayments. This calculator tests each 12 month block of your deal against the percentage you enter.
Can I overpay on a variable rate mortgage?
Terms vary by lender and product. Santander, for example, says overpayments on non-fixed rates are unlimited with no early repayment charge. Check your own mortgage terms.
When does my overpayment start saving interest?
It depends on how your lender works out interest. This calculator assumes interest is worked out monthly, so each overpayment lowers the balance from the next month. Some lenders update daily or yearly.
Is this calculator accurate for interest-only mortgages?
No. It is built for repayment mortgages. Overpaying an interest-only mortgage lowers the balance but the maths differs, so use your lender's figures.
Sources
Checked 20 October 2026:
Lenders set their own overpayment limits and charges. Check your mortgage offer or ask your lender before overpaying.
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