Mortgage Overpayment Calculator
This mortgage overpayment calculator shows what happens when you add a fixed extra amount to every monthly mortgage payment from today onward — how much interest it saves over the life of the mortgage, and how much sooner you'll be mortgage-free. Enter your mortgage details and the extra amount you can afford each month to see the real, compounding impact of consistent overpayments.
Your original mortgage amount, or your current outstanding balance if the mortgage has been running a while.
The rate on your current mortgage deal.
How many years are left on your mortgage.
The additional fixed amount you'll add to every monthly payment, on top of your normal mortgage payment.
Interest saved
£39,951
The total interest you avoid over the mortgage's life by paying more than required every month — e.g. £39,951 saved from a steady £200/month overpayment.
How many years sooner you'll be mortgage-free at this overpayment level, compared to sticking to the required payment only.
How long the mortgage will actually take to clear once the overpayment is included — compare against the remaining term you entered.
Total interest you'll pay with the overpayment included, versus what the mortgage would have cost without it.
How to use this mortgage overpayment calculator
- 1Mortgage balance: your current outstanding balance, not the original loan amount, for the most accurate result if the mortgage has been running a while.
- 2Interest rate and remaining term: use the figures from your current mortgage statement or annual review letter.
- 3Extra monthly overpayment: start with an amount you can sustain every month without strain — even a modest, consistent overpayment compounds into a large saving over a 20+ year mortgage.
- 4Check your lender's overpayment allowance first (commonly 10% of the balance per year on UK fixed deals) to avoid an Early Repayment Charge before committing to a number here.
Understanding your results
Interest saved is the total interest you avoid paying over the life of the mortgage by consistently paying more than required each month. Time saved shows how much sooner the mortgage is paid off — every pound of overpayment goes straight to principal, so the loan balance shrinks faster than scheduled, which snowballs into an increasingly large reduction in future interest. Compare new total interest against your current mortgage's stated total interest to see the full picture side by side.
The formula
Simulated month-by-month: Balanceₙ = Balanceₙ₋₁×(1+r) − (EMI + Extra)Each month, the balance accrues one month's interest and then the original required payment plus your fixed extra amount is subtracted — a larger bite out of the balance every single month than the loan was originally scheduled for. The calculator simulates this month by month until the balance reaches zero, which is the only reliable way to capture how a small, steady overpayment compounds into large savings over hundreds of months, since the effect isn't expressible as one simple formula.
A worked example
A £250,000 mortgage at 4.5% with 25 years remaining, overpaying by £200 every month: the mortgage is paid off about 61 months (just over 5 years) early, saving roughly £39,951 in interest. Double the overpayment to £400 a month on the same mortgage, and the payoff moves forward by 101 months (over 8 years), saving about £63,423 — proportionally more than double the saving from double the overpayment, because the extra principal reduction compounds earlier and earlier as the balance shrinks faster.
Notes for the UK, US and India
In the UK, most mortgage lenders allow overpaying up to 10% of the outstanding balance per year without penalty on fixed-rate deals — check your specific mortgage's allowance, since overpaying beyond it can trigger an Early Repayment Charge (often 1–5% of the excess). Standard variable rate and tracker mortgages typically have no overpayment limit at all. In the US, most conventional mortgages accept unlimited extra principal payments with no penalty — just confirm with your servicer that extra payments are applied to principal, not held as an early next-payment credit. In India, floating-rate home loans carry no prepayment penalty for individuals by RBI regulation, so regular overpayments (locally often called 'part-prepayments') are unrestricted.
Frequently asked questions
How much can I overpay on my mortgage without a penalty?+
Most UK fixed-rate deals allow overpaying up to 10% of the outstanding balance per calendar year penalty-free — check your mortgage offer document or annual statement for your specific allowance, since it varies by lender and product.
Is it better to overpay my mortgage or save the money instead?+
Compare your mortgage rate against the after-tax interest rate you'd earn saving instead. Overpaying a 4.5% mortgage is a guaranteed, tax-free 4.5% return; if your savings account or ISA reliably beats that after tax, saving may be the better choice, but few easy-access accounts do.
Does overpaying reduce my monthly payment or my mortgage term?+
This depends on your lender's policy — most keep your required monthly payment the same and let overpayments shorten the term (which this calculator models, and which saves the most total interest); some lenders let you choose to instead reduce the monthly payment while keeping the original term.
What is a good amount to overpay each month?+
Any consistent amount helps, but the earlier and larger the overpayment, the more it saves, since it compounds against a bigger remaining balance. Even £100–£200 a month on a typical UK mortgage can save tens of thousands of pounds and multiple years — use the calculator above with your own numbers.
Should I overpay my mortgage or pay off other debt first?+
Pay off higher-interest debt (credit cards, personal loans) before overpaying a mortgage — mortgage rates are usually the lowest form of debt you hold, so other debts cost you more per pound and should be cleared first.