Mortgage Overpayment Calculator: Interest Saved and Years Cut

Article updated 27 October 2026

The tool above shows what any overpayment does to your term and total interest. The sections below cover how much you can overpay without a charge, whether a lump sum or a monthly overpayment saves more, and when the money is better placed elsewhere. Mortgage One is a whole of market mortgage adviser helping borrowers weigh overpaying a mortgage against remortgaging, offsetting and saving.

To see whether your money does more as an overpayment or against a sharper rate at renewal, call 01202 155992 or contact Mortgage One.

How the mortgage overpayment calculator works

The calculator applies your overpayment to the mortgage balance each month and recalculates the interest charged on the reduced balance. On £200,000 at 5.05% over 25 years, an extra £100 a month clears the loan 3 years and 8 months early and saves £24,856 in interest.

Every pound you overpay stops being charged interest for the rest of the term, which is why the saving is so much larger than the sum paid in. Mortgage interest is charged on the outstanding balance each month, so a pound overpaid in year two earns its saving for over twenty years, while a pound overpaid in year twenty earns it for five. The monthly payment itself comes from the same amortisation arithmetic as our mortgage repayment calculator, which prices the loan before any overpayment.

Use your current figures, not the original ones. Enter the outstanding balance from your latest statement, the rate you are actually paying and the years left on the term. The tool above models a regular monthly overpayment, a one-off lump sum, or both together, and flags when the total would test a typical 10% annual allowance.

How much can you overpay on your mortgage?

Most fixed rate mortgages allow overpayments of up to 10% of the balance each year without an early repayment charge, so a £200,000 balance carries a £20,000 annual allowance. Trackers and standard variable rate deals are usually unlimited. Overpaying beyond the allowance triggers a charge on the excess, typically 1% to 5%.

The 10% figure hides real differences in how lenders measure it. Some take 10% of the balance on 1 January each year, some 10% of the original advance, and some reset the allowance on the product anniversary rather than the calendar year. A borrower searching for a mortgage overpayment calculator with a 10 percent per year limit is usually asking that measurement question, and the answer sits in the mortgage offer document rather than in any general rule.

The early repayment charge itself usually tapers across the fixed period, stepping down each year from around 5% on a five year fix or 2% on a two year fix. The charge applies only to the amount repaid above the allowance, not to the whole overpayment, but on a large lump sum that excess can still outweigh years of interest saving. Check the schedule in your offer before paying in more than the allowance covers.

Lump sum or monthly overpayments: which saves more?

Pound for pound, money paid earlier saves more, because interest is charged on the balance every month it remains. A £10,000 lump sum at the start of a £200,000 mortgage at 5.05% saves £23,133 in interest and clears the loan 2 years and 5 months early.

Set that against the monthly route and the efficiency gap shows. An extra £100 a month on the same mortgage saves £24,856, slightly more in total, but it takes £25,700 of overpayments spread across 21 years to do it. The £10,000 lump sum achieves almost the same saving with less than half the money, purely because all of it starts working on day one.

In practice the two are not rivals. A lump sum uses allowance in one move, so a windfall larger than 10% of the balance is often best split across two allowance years or timed just before the allowance resets. A monthly overpayment then works quietly inside the allowance in between. Some lenders apply overpayments only at the next anniversary rather than immediately, which costs a year of compounding on every payment, so ask how yours is applied.

Reduce the term or reduce your monthly payments?

Ask the lender to hold your monthly payment and shorten the term, because that is where the saving sits. The same £100 a month on £200,000 at 5.05% saves £24,856 when it shortens the term and only £10,799 when the payment is recalculated downwards instead.

Lenders split on the default. Some automatically shorten the term when you overpay, others recalculate the payment down and leave the term untouched, and most will do either if asked. Reducing the payment gives breathing room in the monthly budget and can make sense when income is tight, but it hands most of the interest saving back. The calculator above models the term reduction route, which is the one that produces the headline savings.

Overpaying also moves your loan to value, which matters at the next renewal. Capital repaid before a remortgage can drop the loan into a cheaper pricing band, and crossing from 80% to 75%, or from 65% to 60%, changes the rate on the whole balance for the next deal. Our loan to value calculator shows how far your balance sits from the next band.

If your fixed deal ends within the next six months and you are weighing overpaying now against a lower loan to value band at renewal, call 01202 155992 or contact Mortgage One.

Should you overpay your mortgage or save instead?

Compare your mortgage rate with the rate your savings earn after tax. With average 2-year fixed mortgage rates at 5.05% in October 2026 and most easy access accounts paying less, overpaying beats saving for many borrowers once an emergency fund is in place.

The comparison is rate against rate, but not only that. Savings stay accessible, while money overpaid into most mortgages cannot be drawn back out without remortgaging or further borrowing. An offset mortgage splits the difference by setting savings against the balance while leaving them accessible, usually at a slightly higher rate. Where mortgage pricing goes next also shapes the answer, and our interest rate predictions UK page tracks where the market prices the base rate over the next five years.

The mortgage rate you are comparing against is the one you will pay next, not necessarily the one you pay now. A borrower sitting on a low fix taken in 2021 may earn more in savings today than the mortgage charges, which reverses the arithmetic until the fix ends. Our table of current UK mortgage rates shows what renewal pricing looks like across the big lenders this month.

When overpaying your mortgage makes less sense

Overpaying makes less sense when more expensive debt is outstanding, when it would empty your emergency fund, or when an early repayment charge of 1% to 5% applies to the excess. Money overpaid into most mortgages cannot easily be taken back out once it is in.

Credit cards, car finance and unsecured loans almost always charge more than a mortgage, so clearing them first saves more per pound and improves affordability for the next mortgage application at the same time. An overpayment also competes with the renewal itself. If the better answer to a high rate is a new deal rather than a smaller balance, our remortgaging guide weighs a product transfer against switching lender on total cost.

Timing matters most in the months before a deal ends. An overpayment made just before a remortgage completes achieves the same loan to value effect as one made a year earlier, without a year of lost access to the money. For anyone planning to move house, repay the mortgage entirely or restructure borrowing, the right sequence is worth a conversation before the money goes in.

For a free initial consultation on whether your figures favour overpaying, offsetting or simply a sharper rate, call 01202 155992 or contact Mortgage One.

Back to Calculators

The information provided in this article is for general guidance only and does not constitute personal or regulated financial advice. If you’d like to understand what these moves could mean for you, speak to Mortgage One. We can explain your options and timings based on your specific circumstances.

FAQs

1. How much can I overpay on my mortgage without penalty? Most fixed rate mortgages allow overpayments of up to 10% of the balance each year without an early repayment charge, and some products allow more. Tracker and standard variable rate deals are usually unlimited. Check your mortgage offer, because lenders measure the 10% against different baselines and reset it at different dates.

2. Should I overpay my mortgage? It depends on your mortgage rate, what your savings earn after tax, whether you hold more expensive debt and how much accessible cash you keep. Overpaying tends to win when the mortgage rate is higher than the savings rate and an emergency fund is already in place. The calculator above shows what your own figures would save.

3. Is it better to overpay my mortgage or reduce the term? Overpaying while holding the monthly payment steady shortens the term anyway, without the commitment. A formal term reduction locks the higher payment in and usually needs an affordability check. Regular overpayments achieve the same interest saving and can be stopped if circumstances change, which is why most borrowers start there.

4. Is it better to overpay monthly or with a lump sum? A lump sum saves more per pound because the whole amount starts reducing the balance immediately. On £200,000 at 5.05%, £10,000 upfront saves £23,133, close to what £25,700 of monthly overpayments saves over the term. Monthly overpayments win on practicality, since few borrowers hold a spare lump sum.

5. Is it worth overpaying £100 a month on my mortgage? Usually, yes. On a £150,000 mortgage at 5.05% over 25 years, £100 a month clears the loan 4 years and 7 months early and saves £23,502 in interest. The smaller the balance, the bigger the proportional effect, because the overpayment is a larger share of each month's capital repayment.

6. Is it better to overpay the mortgage or put money in savings? Compare the mortgage rate with the savings rate after any tax. If the mortgage charges more than savings earn, overpaying produces the better return, with the trade-off that the money is no longer accessible. If your fixed rate is lower than current savings rates, saving until the fix ends can come out ahead.

7. What happens if I overpay more than 10% of my mortgage in a year? The lender applies an early repayment charge to the excess above the allowance, typically between 1% and 5% depending on how far through the fixed period you are. The charge is set out in your mortgage offer. Timing a large overpayment across two allowance years is often enough to avoid it entirely.

8. Can a broker help me decide whether to overpay or remortgage? Yes. A whole of market broker can price the alternatives against each other: overpaying on the current deal, remortgaging to a lower rate or loan to value band, offsetting, or a combination. The right answer depends on your rate, your renewal date and your figures, which is what an initial consultation establishes.