Free Mortgage Overpayment Calculator UK

Last updated: 14 June 2026

I've been using Sprive with my Nationwide mortgage since October 2021, making £100 monthly overpayments through the app. This calculator reflects the same maths that convinced me to start.

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This calculator is for guidance only and does not constitute financial or mortgage advice. CoolCuration is not authorised by the Financial Conduct Authority. Always speak to your lender or a qualified mortgage adviser before acting on the numbers.

Your home may be repossessed if you do not keep up repayments on your mortgage.

How to use this calculator

First, enter your current outstanding mortgage balance. Then add your interest rate and the years left on your term. After that, choose your mortgage type, then either set a one-off lump sum, a recurring monthly overpayment, or both. Finally, hit Calculate.

The mortgage overpayment calculator shows you four things at once:

• How many months or years you would save
• How much interest you would avoid paying
• Your new mortgage-free date
• Your reduced end-of-term balance, if your overpayment doesn't fully clear it in time

If you want to know whether you'd be better off saving the money instead, tick the "compare to savings" box and enter your savings rate. The calculator runs both scenarios and shows you which wins.

Assumptions we've made

In order to calculate these results, we've made a few assumptions:

1. Interest is charged monthly.
2. The interest rate stays the same over the full remaining term. In reality, your rate will change when you remortgage. However, this gives you a like-for-like comparison.
3. If you selected "interest only", we assume your standard monthly payment doesn't decrease, even after you pay off some of the balance.
4. For the savings comparison, we assume you save the same monthly amount and lump sum at a constant savings rate, for the same number of months that your overpayments would take to clear the mortgage. This keeps the time and money commitment equal in both scenarios.
5. We do not account for inflation. Therefore the real-terms value of money saved in the future will be lower than it appears today.
6. We do not account for tax on savings interest. If your savings exceed the Personal Savings Allowance (£1,000 for basic rate, £500 for higher rate), you may owe tax. Mortgage overpayments themselves have no tax implications.

No calculator can predict the future. Rates change, circumstances change, and your mortgage deal will be different in five years. However, this gives you a reliable estimate based on today's numbers, and the direction of travel is clear: even small overpayments make a significant difference over time.

Automate your overpayments with Sprive

Running the mortgage overpayment calculator is step one. Step two is doing it consistently. That's where most people fail.

Sprive automates mortgage overpayments completely. Connect your mortgage, set your amount, and the app handles the rest. There are no fees for the overpayment service and no faff. The app also has a cashback feature that earns you money on everyday shopping, which then goes directly off your mortgage balance.

For transparency: Sprive Limited is an appointed representative of Connect IFA Ltd, authorised and regulated by the FCA (FRN 919863). Money waiting in your Sprive wallet is safeguarded by PrePay Technologies, so it is protected as e-money rather than covered by FSCS. Overpaying still reduces flexibility, because cash sent to your lender is harder to get back than money in an easy-access account.

We've been using Sprive since October 2021. The calculator shows the maths. Sprive does the doing.

Automate your overpayments with Sprive

For our full hands-on opinion after four years of use, read our Sprive mortgage app review.

Why overpaying works

Your mortgage charges interest on the outstanding balance. When you overpay, you reduce that balance. Therefore less interest gets charged the following month. As a result, more of your regular payment goes towards the principal rather than interest. Over time, this compounds.

The first year barely moves the needle. By year five, the gap is visible. By year ten, it's transformative.

For example, on a £200,000 mortgage at 4.5% over 25 years, overpaying £100 a month saves around £21,000 in interest. It also clears the mortgage about three and a half years early. Bigger overpayments produce dramatically bigger savings, and one-off lump sums applied early in your term have the biggest impact of all.

Lump sum or regular overpayment?

Both approaches work. The mortgage overpayment calculator handles either, or both at once. The trade-off is essentially timing versus discipline.

A one-off lump sum applied early in your term has the biggest single impact. Every pound you knock off the balance reduces every interest charge that follows. Therefore the earlier the lump sum, the bigger the compound effect over the remaining years.

A recurring monthly overpayment, on the other hand, builds the habit. It's smaller each month but more sustainable. For most people, £50 to £200 a month is more achievable than finding a £2,500 lump sum on top of normal living.

In practice, the best strategy is usually a blend. Apply a lump sum when you can (a bonus, an inheritance, the sale of a car) and keep a small recurring overpayment running in the background. The calculator above will show you the impact of either approach, or both combined. Before you lock money away, though, keep an emergency fund handy, because overpayments are hard to reverse.

Check your lender's overpayment rules first

Before you trust the figures from any mortgage overpayment calculator, check your specific terms carefully. Most UK lenders allow you to overpay up to 10% of your outstanding balance per year without early repayment charges (ERCs). Go beyond that and you risk a penalty that wipes out your savings.

Some lenders are stricter. Always confirm your exact limit before you start.

If you're on a tracker or standard variable rate, there's usually no overpayment cap. Fixed-rate mortgages almost always have the 10% cap. As an example, Nationwide publishes its overpayment rules clearly, and the structure is fairly typical across major UK lenders.

We've listed the rules for major UK lenders in our Sprive overpayment rules guide. The independent MoneyHelper guidance on mortgage overpayments also covers the basics in plain English. For a wider editorial overview, MoneySavingExpert's pay off your mortgage early guide walks through the strategy in more detail.

Want to go further?

• Our full guide to paying off your mortgage sooner
• How mortgage overpayments work
• When to remortgage alongside overpaying
• If you're remortgaging, a broker does the legwork
• How Sprive's cashback goes straight off your mortgage

Frequently asked questions

How much can I save by overpaying my mortgage?

It depends on your balance, your rate, and how much you overpay. As a rough benchmark, overpaying £100 a month on a £200,000 mortgage at 4.5% over 25 years saves around £21,000 in interest and clears the loan about three and a half years early. Use the calculator above for your own numbers.

How much can I overpay my mortgage per year?

Most fixed-rate mortgages cap penalty-free overpayments at 10% of your outstanding balance each year. Trackers and standard variable rates often have no cap. Always check your specific terms before going over.

Is it worth overpaying my mortgage by £100 a month?

In my experience it usually is. On a typical £200,000 mortgage, £100 a month saves you tens of thousands in interest and clears the loan years early. However, build a three to six month emergency fund first. Money sent to your lender is locked away, so this is not financial advice and your own circumstances matter.

Does overpaying reduce the term or the monthly payment?

For maximum interest savings, most people ask their lender to apply the overpayment to the term, not the monthly payment. Reducing the monthly payment lowers your immediate outgoings but barely shortens the loan. Reducing the term clears the debt faster and saves real money.

Can I get charged for overpaying my mortgage?

Yes, if you go beyond your lender's permitted limit. Most fixed deals charge an early repayment charge of 1% to 5% of the amount overpaid above the 10% allowance. Therefore staying within the limit matters.

What is the 10% overpayment rule?

Most UK fixed-rate mortgages let you overpay up to 10% of your outstanding balance each calendar year without paying an early repayment charge. Some lenders calculate this against the balance at the start of the year. Others use a rolling figure. Always check.

How accurate is a mortgage overpayment calculator?

The maths are accurate based on what you enter, but real life is messier. Rates change at remortgage. Lenders calculate interest in slightly different ways. Treat the result as a strong indication, not a binding contract. For specifics, always confirm with your lender.

More mortgage and money tools

• How the Sprive refer-a-friend bonus works – the structure of the rewards if you bring friends to the app.
• Sprive vs manual overpaying – automation against paying your lender direct, head to head.
• Save thousands on your mortgage UK – tactics beyond overpaying that genuinely move the needle.
• How Sprive moves your money – a deeper look at the mechanics behind the app.
• Chip app guide – an automated savings app for the emergency fund you should build first.

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