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Mortgage calculator

Mortgage Overpayment Calculator

See how much interest you'd save and how many years you'd knock off your mortgage by overpaying a little each month, a one-off lump sum, or both. Free, no signup.

Free, no signupAny currencyInterest + time saved
Your mortgage
What you still owe on the mortgage
£
Your current mortgage rate
%
How long until the mortgage is paid off
yrs
Know your actual payment? Enter it and we'll use it instead of the term, so the result matches your mortgage.
£
Your overpayment
On top of your normal payment
£
A single overpayment now (leave 0 if none)
£
Interest you’d save
£38,458
And you'd be mortgage-free 5 years 1 month soonerPaid off early
10y20y
With overpaymentsWithout
Normal monthly payment£1,390
Interest without overpaying£166,874
Interest with overpaying£128,416
Interest saved£38,458
Time saved5y 1m
Assumes a fixed rate and no overpayment fees or limits. Check your lender’s annual overpayment cap. A guide, not financial advice.
Simon Chadwick
Simon Chadwick
Founder, Orbit Money
Method: month-by-month amortisationUpdated: 13 July 2026Sources: MoneyHelper

Why overpaying saves so much

A mortgage charges interest on the whole outstanding balance, every month, for decades. When you overpay, that extra money comes straight off the balance, so every future month’s interest is calculated on a smaller number. The saving compounds: a modest overpayment early on removes interest that would otherwise have piled up for the entire remaining term. That’s why a little each month can turn into a large saving and a mortgage cleared years early.

Monthly overpayments vs a lump sum

A lump sum cuts the balance immediately, so pound for pound it saves the most interest, it’s working from day one. Regular monthly overpayments are easier to keep up and build momentum over time. The calculator above lets you model either on its own or both together, so you can see which fits your situation. If you come into a windfall, a lump sum plus a small ongoing top-up is often the most powerful combination.

Before you overpay: three checks

  1. Check your overpayment allowance. Many fixed deals cap penalty-free overpayments at 10% of the balance a year; going over can trigger an early repayment charge.
  2. Keep an emergency fund. Money paid into a mortgage is hard to get back, so hold a cash buffer before overpaying.
  3. Clear pricier debt first. Credit cards and personal loans usually cost more than a mortgage, so pay those down before overpaying the mortgage.

How to use this calculator

  1. Enter your outstanding balance, interest rate and the years left on the mortgage.
  2. Add how much extra you could pay each month, a one-off lump sum, or both.
  3. Read the interest you'd save and how much sooner you'd be mortgage-free.
  4. Try different overpayment amounts to find a level you can comfortably keep up.

Frequently asked questions

How much do mortgage overpayments save?
It depends on your balance, rate and how much you overpay, but the effect is usually bigger than people expect because you cut interest for the whole remaining term. On a £250,000 mortgage at 4.5% with 25 years left, overpaying £200 a month can save tens of thousands in interest and clear the mortgage several years early. Enter your own figures above to see your saving.
Is it better to overpay monthly or with a lump sum?
Both help, and the calculator lets you model either or both. A lump sum cuts the balance straight away, so it saves the most interest per pound. Regular monthly overpayments build up over time and are easier to sustain. Many people do a bit of both: a lump sum when they can, plus a small monthly top-up.
Is there a limit on how much I can overpay?
Often yes. Many fixed-rate mortgages let you overpay up to 10% of the balance each year without a penalty; go over that during the fixed period and you may face an early repayment charge. Once you're on a variable rate there's usually no limit. Always check your specific mortgage terms before overpaying a large amount.
Should I overpay my mortgage or save the money instead?
Compare your mortgage rate with the interest you could earn on savings or investments after tax. If your mortgage rate is higher than a guaranteed savings rate, overpaying usually wins because it's a risk-free return equal to your mortgage rate. Keep an emergency fund first, and clear higher-interest debt like credit cards before overpaying a mortgage.
Does overpaying reduce my monthly payment or my term?
By default most lenders keep your monthly payment the same and shorten the term, which is what this calculator assumes and what saves the most interest. Some lenders let you instead reduce your monthly payment and keep the term, which frees up cash flow but saves less. Ask your lender which option applies when you overpay.

Related tools

Compound Interest Calculator
See how the same money could grow if you saved it instead.
50/30/20 Budget Calculator
Work out how much you can afford to overpay each month.
Net Worth Calculator
See how paying down the mortgage lifts your net worth.

Overpaying frees up money. So does cutting what you forgot.

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Simon Chadwick
About the author
Simon Chadwick
Founder of Orbit Money

Simon is the founder of Orbit Money, a tool that helps people track subscriptions and recurring spend. He builds Orbit's free money calculators and writes about personal finance for UK and Australian readers.

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This tool is a guide, not financial advice.