Mortgage Overpayment Calculator
See how much interest you'd save — and how many years you'd shave — by overpaying.
Your mortgage interest saved
Add a monthly amount or lump sum and see how much interest you'd dodge.
How it works
This calculator shows what happens if you pay a little extra towards your mortgage — as a regular monthly top-up, a one-off lump sum, or both. It tells you how much sooner you'd be mortgage-free and how much total interest you'd avoid over the remaining term.
The reason overpayments are powerful is compounding in reverse: interest is charged on whatever balance is left, so every extra pound paid today removes interest from every future month. Pound for pound, an overpayment made in year one is worth far more than the same overpayment made in year twenty.
It's most useful for borrowers a few years into a fixed deal who suddenly have spare cash flow — a pay rise, a smaller energy bill, a finished car loan. It's less useful if you're carrying higher-cost debt elsewhere (credit cards, overdrafts) or don't yet have an emergency fund; in both those cases your money usually works harder somewhere else first.
A worked example
Take a £200,000 mortgage at 5% with 25 years remaining. The standard monthly payment is around £1,169. Add £200 a month on top and you'd clear the mortgage roughly six years earlier and save in the region of £55,000 in interest over the term. A single £10,000 lump sum paid today, with no other changes, would save a further £15,000–£20,000 by the time the term ended — because that £10,000 stops attracting interest for the next 25 years.
Why this matters
For most households, the mortgage is the largest debt they'll ever hold and the largest single line on the monthly budget. Even modest overpayments — the cost of a takeaway a week — meaningfully change both the date you finish paying and the total interest bill. Crucially, the "return" on an overpayment is guaranteed and equal to your mortgage rate, which is one of the few risk-free returns most people can access.
Common mistakes
- Overpaying without checking the early repayment charge cap on a fixed deal (commonly 10% of the balance per year).
- Overpaying before building a 3–6 month emergency fund — once the money's in the mortgage, it's hard to get back without remortgaging.
- Overpaying while carrying credit card or personal loan debt that costs more than the mortgage rate.
- Asking the lender to lower the monthly payment when shortening the term would save far more in total interest.
- Forgetting to tell the lender it's an overpayment — some lenders treat extra payments as advance payments unless you specify.
Beyond the numbers
A sensible overpayment plan usually sits inside a wider order of priorities: a working emergency fund first, then any higher-rate debt cleared, then long-term tax-efficient investing (workplace pension, ISA), with overpayments competing alongside the last of those. Within the mortgage itself, the two choices that matter most are the early repayment charge limit on your current deal and whether to shorten the term or reduce the monthly payment when you overpay. Shortening the term keeps the budget the same and compounds the benefit; reducing the payment frees up cash flow now but gives back much of the saving. If the choice feels close, a partial split — overpay some, keep some flexibility — is usually fine. See the Mortgage Calculator to compare the resulting monthly payment under each scenario.
Related tools: Mortgage Calculator · Mortgage Affordability · House Deposit · Savings Goal
Frequently asked
Complete Guide
UK Mortgage Overpayment Guide: How Much Could You Save?
Owning a home is often the biggest financial commitment you'll ever make, and the mortgage is a cost that sticks around for decades. The good news: small overpayments can cut years off the term and reduce the total interest you pay. This UK guide breaks down how mortgage overpayments work, how much you could realistically save with a worked example, and when overpaying may not be the right move.
What is a mortgage overpayment?
In plain English, a mortgage overpayment is any money you pay towards your mortgage on top of your required monthly instalment.
When you make your normal monthly payment, a significant portion goes towards the interest the bank charges you for the loan. Only a smaller portion actually reduces the amount you borrowed (the "principal").
When you overpay, 100% of that extra money goes directly toward reducing the principal. Because the principal is now smaller, the bank has less debt to charge interest on next month. It's a snowball effect that can significantly reduce the total cost of your home.
There are two main ways to overpay: monthly overpayments, where you add a set amount (say, £100) to your direct debit every month; and lump-sum overpayments, where you pay a one-off amount (perhaps a bonus or inheritance) directly into your mortgage account.
Why overpaying can make a big difference
Think of your mortgage interest like a leak in a bucket. The more debt you have, the bigger the leak. Overpaying helps plug that leak.
Many UK mortgages calculate interest daily. That means as soon as your overpayment hits the account, you reduce the future interest charged on that portion of the balance.
A simple way to think about the benefit: overpaying gives you a return roughly equal to your mortgage rate. If your rate is 5%, each pound used to reduce the balance avoids interest at that rate. For many borrowers, that makes overpaying an efficient use of spare cash.
That said, lender terms vary. Some mortgages limit overpayments and early repayment charges may apply, so always check the details of your specific deal first.
See the maths: a worked example
Numbers can feel abstract until you see them applied to a real house. Take a typical UK scenario: a £250,000 mortgage balance at a 5% interest rate over a 25-year remaining term, with a standard monthly payment of about £1,461.
If you simply paid this amount for 25 years, you would end up paying back a total of around £438,443.
Now imagine you added £200 extra every month. The total repaid drops to around £394,011, you save about £44,432 in interest, you cut roughly 5.2 years (62 months) off the term, and your new payoff term is around 19.8 years.
Your own numbers will differ — try the Calcaroo Mortgage Overpayment Calculator to model your exact balance, rate and overpayment.
Monthly vs lump sum: which is better?
If you have cash sitting in a savings account, you might wonder whether it's better to drop it all in at once or drip-feed it monthly.
Monthly overpayments are simple and consistent. You adjust your budget once, and the savings build in the background. The downside is you don't get the immediate interest impact of a larger lump sum.
Lump-sum overpayments tend to save more interest overall, because interest is often calculated daily — a £5,000 payment made today usually saves more than the same £5,000 spread over several years. The trade-off is that the money is much harder to get back if you need it.
If you're unsure, run both scenarios on our mortgage overpayment tool. A mix of the two often works well in real life.
When overpaying may not be the best option
Overpaying isn't a one-size-fits-all solution. Four common situations where keeping the cash elsewhere may be the better call:
1. You don't have an emergency fund. Never overpay if it leaves you short on accessible savings. Money paid into the mortgage usually can't be pulled back out quickly. Aim for 3–6 months of essential expenses in an easy-access account first.
2. You have high-interest debt. If a credit card is charging 22% interest, clearing that almost always beats overpaying a 5% mortgage. Start with the most expensive debt first — you can compare scenarios with the Calcaroo Credit Card Payoff Calculator.
3. Early repayment charges (ERCs) apply. Many fixed-rate UK mortgages limit overpayments to around 10% of the balance per year. Exceed that and the lender may charge an ERC, which can wipe out some or all of the benefit.
4. Better returns elsewhere. If your mortgage rate is low and you can earn more after tax in savings or use the money more effectively, overpaying may not be the best choice. Run both options side by side before deciding — the Loan Comparison Calculator can help when weighing different borrowing trade-offs.
3 common overpayment mistakes to avoid
Assuming it's unlimited. Always check your mortgage offer. Some lenders reset the overpayment allowance on 1 January, others use the anniversary of your mortgage start date. Don't guess — check your latest statement.
Ignoring the term vs payment choice. When you overpay, some lenders keep the term the same and reduce your future monthly payment instead. To save the most interest, you usually want to ask them to keep payments the same and shorten the term.
Forgetting to tell the lender. On some accounts a plain bank transfer may not automatically be treated as an overpayment. Always check how your lender wants overpayments to be made and applied.
Ready to run your numbers?
The best way to decide if overpaying is right for you is to compare the numbers for your own mortgage. Pop in your balance, rate, and term to see how much interest and time you could save — it's fast, free, and no signup required.