---
title: "Mortgage Overpayment Calculator (UK): Pay Off Your Mortgage Early"
description: "See how monthly or lump-sum overpayments change your mortgage-free date and the interest you pay, from today's balance. Checks your 10% allowance and early repayment charge."
url: https://ownedoutright.com/uk/mortgage-overpayment-calculator/
---

UK mortgage · Overpayments

# Mortgage overpayment calculator

Start from what you owe today, add a monthly or one-off overpayment, and see your new mortgage-free date, the interest you save, and whether you stay inside your lender's overpayment allowance.

By [Ethan Brooks](https://ownedoutright.com/about/) · Updated October 15, 2026 · [How we calculate](https://ownedoutright.com/methodology/)

## What an overpayment actually does

Every monthly payment first covers the interest charged on your balance. Whatever is left reduces the capital. An overpayment skips the interest step: all of it comes off the balance, so there is less to charge interest on from then until the mortgage ends. That is why a modest overpayment early in a 25-year mortgage saves far more than it looks, and why the same money matters less in the final years, when the balance is already small.

## How much you save: worked examples

Take a £200,000 repayment mortgage at 4.5% over 25 years. The payment is £1,111.66 a month and the interest over the full term is £133,501. Here is what common overpayments do to it:

| Overpayment | Mortgage-free sooner by | Interest saved | | --- | --- | --- | | £100 a month | 3 yrs 6 mo | £21,142 | | £200 a month | 6 yrs 1 mo | £36,281 | | £500 a month | 11 yrs | £63,888 | | £10,000 once, in year one | 2 yrs 1 mo | £18,681 |

Calculated with the same engine as the calculator above. Monthly overpayments start with the first payment.

## The 10% allowance and early repayment charges

Most fixed and discounted deals let you overpay a set share of the mortgage each year, usually 10%, without any charge. Overpay more than that in a deal year and the lender charges an early repayment charge (ERC) on the excess only, typically a few per cent of it. On the £200,000 example with a 10% allowance that is £20,000 in year one, or about £1,667 a month. A £30,000 lump sum in that year would be £10,000 over the allowance; at a 3% charge, that costs £300. The calculator checks each deal year up to the date your deal ends and shows any charge next to the interest you save, so you can see the net figure. Our guide to the [10% overpayment rule and early repayment charges](https://ownedoutright.com/uk/guides/10-percent-overpayment-rule/) explains which balance the allowance is taken from and how to overpay more without paying it.

## Reduce the term or reduce the payment?

When you make a lump-sum overpayment, most lenders ask which you want. Keeping your payment the same shortens the term and saves the most interest: on the example, a £10,000 lump sum in June 2027 finishes the mortgage 2 yrs 1 mo sooner and saves £18,681. Asking for a lower payment keeps the original end date and drops the payment from £1,111.66 to £1,055.47 a month, which can be the better choice if your budget is tight or your rate is low. Regular monthly overpayments normally leave the payment unchanged and shorten the term; the calculator treats them that way.

## Start from the mortgage you have today

Many overpayment calculators assume a brand-new mortgage: the amount borrowed, a full 25 years, and overpayments from the first month. If you are five or ten years in, that overstates what is left to save. This calculator starts from your current balance and your next payment date. Enter either the term you have left or your monthly payment and it works out the other. Your balance is on your annual statement or in your lender's app.

## Before you overpay, check these

- **Your deal's terms.** The allowance, the early repayment charge and the date your deal ends are in your mortgage offer. Some lenders count the deal year from the day the deal started, others use the calendar year.
- **How the money is applied.** Tell your lender it is an overpayment to reduce the capital, and whether you want the term or the payment reduced. Check the next statement.
- **Your other money.** An emergency fund of a few months' spending, any high-interest debt, and employer pension contributions you are not yet claiming usually come first. Our guide [should you overpay your mortgage or save?](https://ownedoutright.com/uk/guides/overpay-mortgage-or-save/) walks through the comparison, including savings interest and ISAs.
- **Your deal ending.** Overpaying before a fixed rate ends lowers the balance the new payment is worked out on. The [fixed rate ending calculator](https://ownedoutright.com/uk/fixed-rate-ending-calculator/) shows the payment after your deal at the rate you expect.
- **Getting the money back.** Overpayments are not usually refundable, although some lenders offer a reserve or a payment holiday against earlier overpayments. If you may need the money, savings are more flexible.

## How this calculator works

Interest each month is your balance times the annual rate divided by 12, rounded to the penny. Your scheduled payment pays that interest first; the rest, plus any overpayment, reduces the balance. Overpayments are credited after that month's payment, so they lower interest from the next month. Most UK lenders calculate interest daily, which gives a result within a few pounds of this. Insurance, fees and any product fee added to the mortgage are not included. The early repayment charge is applied to the amount above your allowance in each deal year. The full method is on [How we calculate](https://ownedoutright.com/methodology/).

## Related tools and guides

- [Fixed rate ending calculator](https://ownedoutright.com/uk/fixed-rate-ending-calculator/): your payment after the deal ends, and what overpaying now does to it.
- [The 10% overpayment rule and early repayment charges](https://ownedoutright.com/uk/guides/10-percent-overpayment-rule/).
- [Should you overpay your mortgage or save?](https://ownedoutright.com/uk/guides/overpay-mortgage-or-save/)
- [What happens when your fixed rate ends](https://ownedoutright.com/uk/guides/what-happens-when-your-fixed-rate-ends/).

## Questions people ask

### How much can I overpay on my mortgage without a penalty?

It depends on your deal. Most fixed and discounted deals let you overpay up to a set share of the mortgage each year, commonly 10%, without an early repayment charge. Which balance the 10% is taken from (the balance at the start of each deal year, or the amount you originally borrowed) is in your mortgage offer. On a £200,000 balance with a 10% allowance, that is £20,000 in the first year, or about £1,667 a month. On a standard variable rate or most trackers there is usually no limit.

### What happens if I overpay more than 10%?

The lender charges an early repayment charge on the amount above the allowance, not on the whole overpayment. A £30,000 lump sum in year one of the example, with a 10% allowance and a 3% charge, is £10,000 over the allowance, so the charge is £300. Splitting the same money across two deal years avoids it. The calculator checks every deal year for you.

### Should I reduce the term or reduce my monthly payment?

Reducing the term saves more interest, because your payment stays the same and the mortgage ends sooner. Reducing the payment keeps the end date and lowers what you pay each month, which helps if money is tight. After a £10,000 lump sum on the example mortgage, keeping the payment finishes 2 yrs 1 mo sooner and saves £18,681; lowering the payment instead takes it from £1,111.66 to £1,055.47 a month. Choose "reduce my payment" under the lump sum to see both.

### Does overpaying reduce the interest straight away?

With most UK lenders, yes. They calculate interest daily on the balance you owe, so an overpayment starts saving interest the day it is applied. A few older mortgages calculate interest monthly or annually, so an overpayment only counts from the next calculation date; your statement or lender will say which. This calculator works month by month, which is within a few pounds of a daily calculation.

### Is it better to overpay monthly or in a lump sum?

The same money paid sooner saves more interest, so a monthly overpayment from now slightly beats saving it up for a lump sum at the end of the year. The difference is small; the habit you keep matters more. A lump sum early in the mortgage, when the balance is largest, is the most powerful single overpayment.

### Can I use this if I have had my mortgage for years?

Yes. Enter the balance from your latest statement or your lender's app, your current rate, and either the years left or your monthly payment. The calculator starts from that point, not from the day you took the mortgage out.

For education and estimates only; not financial, tax or legal advice. Your lender's figures are final. See [How we calculate](https://ownedoutright.com/methodology/).

Source: https://ownedoutright.com/uk/mortgage-overpayment-calculator/