ESTIMATED INTEREST SAVED
£0.00
Enter your mortgage details to calculate.
This estimate assumes a repayment mortgage, a constant interest rate, monthly interest and the same scheduled monthly repayment throughout. It excludes lender fees, overpayment limits and early repayment charges. Check your mortgage terms before making a payment.
How mortgage overpayments work
A mortgage overpayment is an amount paid in addition to your scheduled repayment. On a repayment mortgage, reducing the outstanding balance can reduce future interest and bring forward the date when the mortgage is paid off. This calculator estimates those effects for a regular monthly overpayment.
Enter your current balance, annual interest rate, remaining term and proposed extra monthly payment. The calculator first estimates the scheduled repayment, then works through the remaining balance one month at a time. It compares the original schedule with the schedule including your overpayment.
The chart shows how the balance falls in each case. Add up to two other monthly overpayment amounts to compare them on the same time axis. The balance, interest rate, original term and scheduled monthly repayment remain fixed, so each difference is caused by the extra payment entered.
Worked example: £200,000 balance and £200 extra each month
Suppose a repayment mortgage has £200,000 outstanding, 25 years remaining and an illustrative annual interest rate of 5%. The scheduled monthly repayment is approximately £1,169.18. If that payment remains unchanged and an additional £200 is paid every month, the model gives:
| Scheduled monthly repayment | £1,169.18 |
| Additional monthly overpayment | £200.00 |
| Interest without overpayments | £150,754.02 |
| Interest with overpayments | £108,911.43 |
| Estimated interest saved | £41,842.60 |
| Estimated time saved | 74 monthly payments, or 6 years and 2 months |
These figures assume the 5% rate continues for the whole calculation and that the scheduled repayment does not fall after the overpayment. The final payment may be smaller than a normal monthly payment. Actual lender calculations and contract terms can produce different results.
Why the timing of an overpayment matters
Interest is calculated on the outstanding balance in this model. Paying extra earlier reduces the balance on which later interest is calculated. The same monthly overpayment can therefore have a different estimated effect depending on the starting balance, rate and remaining term.
The chart makes that change visible: the lines begin at the same balance, then separate as each extra payment reduces one scenario faster. Each line ends when its mortgage reaches zero. The x-axis remains anchored to the original term so the payoff dates can be compared directly.
Comparing monthly overpayment amounts
Your main overpayment is shown alongside the original schedule. Use the plus buttons for up to two alternative monthly amounts. Each completed comparison recalculates when you change a main mortgage input; an incomplete comparison does not produce a chart line or a zero-looking result.
For a separate decision about changing the mortgage interest rate or replacing your deal, use the Remortgage Calculator. A lower rate, product fees and early repayment charges need to be considered together; changing the rate inside an overpayment row would obscure what caused the difference.
What this calculator includes and excludes
The estimate uses the outstanding balance, a single annual interest rate, the remaining term entered in years and months, and a fixed extra payment made every month. It models a standard repayment mortgage with interest applied monthly.
It does not automatically include:
- changes in the interest rate or a new mortgage deal
- lender changes to the scheduled repayment after an overpayment
- one-off lump-sum overpayments or irregular extra payments
- annual overpayment allowances or early repayment charges
- mortgage arrangement, valuation or legal fees
- interest-only mortgage arrangements
- daily-interest methods or lender-specific rounding
Your lender may apply overpayments differently or recalculate your scheduled payment. Check whether an overpayment will reduce your monthly repayment, shorten your term, or require you to request a particular treatment. The Mortgage Calculator can show how the scheduled payment changes with the mortgage amount, rate and term.
Overpayment limits and early repayment charges
Some mortgage deals limit the amount you can overpay without a charge, particularly during a fixed-rate period. A charge can reduce or outweigh the saving shown by a simple interest calculation. Your mortgage agreement and lender can confirm the applicable allowance and any charge before you act.
If you are considering a broader change to your mortgage, the Mortgage Cost Awareness Calculator can help you think about costs beyond the scheduled repayment.
Important note
This page provides an illustrative calculation, not personalised mortgage or financial advice. It assumes the entered interest rate stays constant throughout the remaining term. Check your lender’s current terms, fees and payment treatment before deciding whether or how much to overpay.
Frequently Asked Questions
Does an overpayment always shorten the mortgage term?
This calculator holds the scheduled monthly repayment fixed, so an extra payment brings the estimated payoff date forward. A lender may instead recalculate your scheduled repayment. Confirm how your mortgage provider handles extra payments.
Does the calculator allow a £0 overpayment?
Yes. A £0 overpayment shows the original repayment schedule and £0 estimated interest saved.
What happens if I overpay enough to clear the mortgage quickly?
The calculation caps the final payment at the amount still owed. The corresponding chart line ends at its payoff month, while the original term remains visible for comparison.
Can I enter a lump-sum overpayment?
No. This calculator models the same additional amount paid every month. A one-off payment has a different timing and is not represented by the monthly overpayment field.
Are mortgage rates assumed to remain unchanged?
Yes. The entered annual rate is held constant for the full calculation. An actual fixed rate may end before the mortgage does, while variable or tracker rates may move.
Are early repayment charges included?
No. The displayed interest saving does not subtract lender fees or early repayment charges. Check your mortgage agreement and ask your lender about the specific payment you intend to make.
Can I compare remortgage offers here?
This chart isolates changes in monthly overpayment at one interest rate. The Remortgage Calculator is the more relevant starting point for comparing an existing mortgage with a replacement deal.
Does this work for interest-only mortgages?
No. It uses a repayment-mortgage schedule in which regular payments reduce the balance. An interest-only mortgage has a different repayment structure.
Can a larger overpayment save less than a smaller one?
Under this calculator’s fixed-rate assumptions, a larger regular overpayment should not increase total interest. In practice, charges or a lender’s treatment of payments can change the overall financial result.
Sources and review
The calculation and supporting explanation were reviewed in September 2026. For guidance on mortgage overpayments, remortgaging and the terms that may affect an actual saving, see:
- MoneyHelper – should you pay off your mortgage early?
- MoneyHelper – remortgaging to cut costs
- Financial Conduct Authority – mortgage and interest rate information
This is a mathematical estimate based on the inputs and assumptions stated above. Mortgage products and lender terms can change, so check current information before making a financial decision.