Overpaying a mortgage is unglamorous, but the maths is striking: a modest monthly overpayment can remove years from your term and save tens of thousands in interest. This calculator shows exactly what a regular overpayment — or a one-off lump sum — does to a UK mortgage.
Mortgage Overpayment Calculator
See how much interest you’d save — and how much sooner you’d be mortgage-free — by overpaying every month.
How it works: your contractual monthly payment stays the same and the extra amount comes straight off the balance, so less interest builds up and the loan clears early. This assumes your rate stays fixed for the whole term and that your lender lets you use overpayments to shorten the term — check for early repayment charges and your lender’s annual overpayment allowance (often 10% of the balance) before overpaying.
How it works
Enter your outstanding balance, your current interest rate, and the years remaining on your term. Then add the extra amount you could pay each month, or a lump sum you are considering paying off in one go. The calculator compares your mortgage with and without the overpayment, showing the new payoff date, the shortened term, and the total interest saved.
You can also model a remortgage scenario by changing the interest rate — useful if you are weighing whether overpaying your current deal beats saving the money elsewhere.
How the calculation works
Interest on a UK mortgage is calculated on the outstanding balance, usually daily. Every overpayment reduces that balance immediately, so every following month’s interest charge is slightly smaller. Your contractual payment stays the same, which means a bigger slice of it clears capital rather than interest — a snowball effect that accelerates as the balance falls.
The new term is found by solving the amortization formula for the number of payments at your higher monthly amount. Two cautions shape real-world results. First, most fixed-rate deals cap penalty-free overpayments at 10% of the balance per year; exceeding the cap triggers an early repayment charge (ERC), often 1–5% of the amount overpaid. Second, lenders apply overpayments differently: some shorten your term automatically, others reduce your monthly payment unless you instruct them otherwise. For maximum benefit, you want the payment kept the same and the term shortened.
Worked example: £200,000 balance at 4.5%, 25 years remaining
Step 1 — Normal payment. At 4.5% over 300 months, the standard payment is about £1,112 per month. Total repaid is roughly £333,500, so total interest is about £133,500.
Step 2 — Add a £200 monthly overpayment. You now pay £1,312 per month. Solving the formula with the higher payment gives a term of about 227 months instead of 300.
Step 3 — The saving. Your mortgage ends roughly 6 years and 2 months early. Total repaid falls to about £297,200, so interest drops to roughly £97,200 — an interest saving of around £36,000 in exchange for £200 a month you may barely miss. Check the 10% allowance: 10% of £200,000 is £20,000 a year, and £200 a month is only £2,400 a year, comfortably inside the limit.
Things to watch
- Check your overpayment allowance before paying a lump sum. On a fixed deal, breaching the 10% annual cap can cost more in ERCs than the interest you save. Tracker and standard variable rate mortgages usually allow unlimited overpayments.
- Tell your lender how to apply the money. If they reduce your monthly payment instead of your term, the interest saving shrinks dramatically. Most lenders let you set “reduce term” as the default in their app or by phone.
- Keep an emergency fund first. Money paid into a mortgage is hard to get back — offset mortgages and agreed payment holidays aside. Three to six months of expenses in an easy-access account should come before aggressive overpaying.
- Compare against your savings rate after tax. Overpaying at 4.5% earns a guaranteed, tax-free 4.5% return. If a savings account pays 4% and you pay tax on the interest, the mortgage usually wins; if you have ISA allowance and a higher rate, the answer can flip.
- Clear expensive debts first. A credit card at 24% APR grows far faster than a mortgage at 4.5% shrinks. Overpaying the mortgage while carrying card debt is moving money in the wrong direction.
Frequently asked questions
Is it better to overpay monthly or save up a lump sum?
Monthly overpayments win on pure maths, because each payment starts saving interest immediately rather than waiting in a savings account earning less. Lump sums suit irregular income, such as bonuses or commission.
Can I overpay during a fixed-rate deal?
Usually yes, up to your annual allowance — commonly 10% of the outstanding balance, though some lenders now allow 20% or calculate it from the original loan amount. Your mortgage offer and annual statement state the exact figure.
Should I shorten my term instead of overpaying?
Shortening the term at remortgage raises your contractual payment and gives the same interest benefit, but the higher payment is compulsory. Overpaying on a longer term is voluntary month by month, which many households prefer as insurance against a drop in income.
What is an offset mortgage and how is it different?
An offset links your savings to your mortgage: interest is charged only on the balance minus your savings. Your money stays accessible, unlike an overpayment, but offset rates are often slightly higher and the benefit depends on keeping substantial savings in the account.
Will overpaying affect my credit score?
Positively, if anything. You are paying more than the contractual minimum, which lenders view favourably. It will not help if you then miss a required payment, so keep overpayments separate from the minimum due.
This calculator is for guidance only and isn’t financial advice.