UK Mortgage Repayment Calculator
Estimate your normal monthly mortgage payment and total interest before overpaying.
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Mortgage Overpayment Calculator
Estimate how much interest you could save and how much sooner you could repay your mortgage by making monthly or one-off overpayments.
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- ✓Interest saved and months saved estimate
- ✓Monthly overpayments and one-off lump sums
- ✓Assumes overpayments reduce the term
Calculate your mortgage overpayment savings
Enter your balance, rate, remaining term and overpayment amount. The calculator estimates the new payoff time and interest saved.
How to use this calculator
- Enter your current mortgage balance and interest rate.
- Add the remaining mortgage term in years.
- Enter the extra monthly amount you want to pay.
- Add any one-off lump sum overpayment.
Overpayment formula
standard_payment =
repayment_formula(balance, rate, term)
baseline_interest =
standard_payment × remaining_months - balance
new_balance = balance - one_off_overpayment
Each month:
interest = balance × monthly_rate
balance = balance + interest
payment = standard_payment + monthly_overpayment
balance = balance - payment
interest_saved =
baseline_interest - overpayment_interest
The calculation assumes your normal monthly payment stays the same and the overpayment reduces the mortgage term.
Estimated interest saved
£41,842.60
Estimated reduction in interest compared with making no overpayments.
Time saved
6 years, 2 months
Estimated reduction in your remaining mortgage term.
New payoff time
18 years, 10 months
Estimated time to clear the mortgage with your selected overpayments.
Standard monthly payment
£1,169.18
Estimated normal repayment before any monthly overpayment is added.
Total overpaid
£45,200.00
Estimated extra amount paid through monthly and one-off overpayments.
£200 monthly extra
Term reduced
This calculator does not include early repayment charges. Check your lender’s overpayment allowance before paying extra.
Ready to act on your result?
If overpaying saves less than switching to a better deal, a remortgage comparison may be worth checking before you pay extra.
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What your overpayment result means
The interest saved figure compares your mortgage with and without the selected overpayments. It shows how much interest could be avoided by reducing the balance faster.
The time saved result shows how much earlier the mortgage could be cleared. This calculator assumes the lender keeps your normal monthly payment unchanged and uses the overpayments to shorten the term.
Total overpaid shows the extra money paid above your normal monthly mortgage payments. This includes the one-off overpayment and the monthly overpayments made until the mortgage is cleared.
Check before overpaying: some mortgage products limit how much you can overpay without an early repayment charge. Your lender may also ask whether you want to reduce the term or reduce future monthly payments.
How mortgage overpayments work
A mortgage overpayment is any extra amount you pay above your normal monthly repayment. It can be a regular monthly amount, a one-off lump sum, or a mixture of both.
On a repayment mortgage, overpayments usually reduce the outstanding balance faster. Because interest is charged on the remaining balance, a lower balance means less interest can build up in future months.
Overpayments can be applied in different ways. If the lender keeps your normal payment the same, the mortgage is usually paid off sooner. If the lender recalculates your monthly payment, the monthly cost may fall instead, but the mortgage term may not reduce as much.
The biggest savings usually come when you overpay early, when the balance is larger and there is more time for interest savings to build. A lump sum at the start can therefore have a stronger effect than the same amount paid much later.
The main risk is liquidity. Money used to overpay the mortgage is not always easy to access again. It is usually sensible to keep an emergency fund and check for early repayment charges before making large overpayments.
What affects your mortgage overpayment saving
- Mortgage balance: A larger balance gives overpayments more room to reduce future interest.
- Interest rate: The higher the rate, the more valuable each pound of balance reduction can be.
- Remaining term: More years remaining means more time for interest savings to build.
- Monthly overpayment: Regular extra payments steadily reduce the balance and can shorten the term.
- One-off overpayment: A lump sum can reduce the balance immediately and may save interest quickly.
- Overpayment charges: Early repayment charges can reduce or wipe out the benefit of overpaying.
- How your lender applies it: Reducing the term usually saves more interest than reducing the monthly payment.
Mortgage overpayment calculator FAQs
Does overpaying my mortgage save interest?
Yes, overpaying a repayment mortgage usually reduces the balance faster. A lower balance means less interest can be charged in future, which can reduce the total cost of the mortgage.
Will overpaying reduce my monthly payment or shorten the term?
This calculator assumes your normal monthly payment stays the same and the overpayment shortens the term. Some lenders may reduce future monthly payments instead, so check how your lender applies overpayments.
Can I overpay my mortgage without a charge?
It depends on your mortgage product. Many fixed-rate and introductory deals have overpayment limits, and early repayment charges may apply if you exceed the allowance.
Is it better to overpay monthly or make a lump sum payment?
Both can work. A lump sum reduces the balance immediately, while monthly overpayments reduce the balance steadily. The best option depends on your cash flow, lender rules and whether you need access to the money.
Should I overpay my mortgage or save instead?
Compare your mortgage rate — especially if you are on a standard variable rate — with the after-tax return you could get from savings. Also consider emergency savings, pension contributions, early repayment charges and whether you may need the cash later.
Does this calculator include early repayment charges?
No. The result shows the estimated interest and time saving before charges. Check your mortgage offer, online account or lender support team before making a large overpayment.
Key terms used in this calculator
These terms explain the mortgage concepts behind the calculation.