What Happens If You Overpay Your Mortgage by £100, £200 or £500 a Month?
Most mortgage overpayment advice stops at “it saves you thousands”. That is true but not very useful: thousands over how long, at what cost, and compared with what? This guide runs the actual numbers on a typical £200,000 UK mortgage at 4.5% over 25 years, so you can see exactly what £100, £200 or £500 a month does to your interest bill and your mortgage-free date, how the answer changes with your rate and balance, and the one setting that can halve the benefit if you leave it on the lender’s default.
Published: 23 September 2026 at 09:00 · 8 min read
The Short Answer
On a £200,000 mortgage at 4.5% over 25 years, the standard payment is £1,112 a month and you pay about £133,500 of interest over the full term. Overpaying from day one, with the lender shortening the term rather than reducing the payment:
- £100 a month clears the mortgage in 21.5 years, 3.5 years early, and saves about £21,100 of interest.
- £200 a month clears it in 18.9 years, 6.1 years early, and saves about £36,300.
- £500 a month clears it in 14.0 years, 11 years early, and saves about £63,900.
The rest of this article explains where those figures come from, why the first £100 is worth proportionally more than the next £400, and what happens when you change the rate, the balance, the start date or the way the lender applies the overpayment.
How Much Does Overpaying £100, £200 or £500 a Month Save?
The table below covers the full range from £50 to £1,000 a month on the same £200,000, 4.5%, 25-year mortgage. “Total overpaid” is the sum of every extra pound you put in before the loan clears, which is the honest cost of the saving.
| Monthly overpayment | Mortgage cleared in | Years cut | Interest saved | Total overpaid |
|---|---|---|---|---|
| £0 (baseline) | 25.0 years | – | – (£133,500 paid) | £0 |
| £50 | 23.2 years | 1.8 | £11,500 | £13,900 |
| £100 | 21.5 years | 3.5 | £21,100 | £25,800 |
| £200 | 18.9 years | 6.1 | £36,300 | £45,400 |
| £300 | 16.9 years | 8.1 | £47,700 | £60,900 |
| £500 | 14.0 years | 11.0 | £63,900 | £84,000 |
| £750 | 11.5 years | 13.5 | £77,000 | £103,500 |
| £1,000 | 9.8 years | 15.2 | £86,000 | £118,000 |
Two things stand out. First, the returns diminish. £100 a month saves £21,100, but the second £100 adds only £15,200, and going from £200 to £500 adds £27,600 for two and a half times the outlay. Each extra pound is applied to a mortgage that the previous pounds have already shrunk, so there is less interest left to remove. Second, the “saving” is not free money. To save £36,300 with £200 a month you hand over £45,400 earlier than you otherwise would. The right way to read the table is as a return on those pounds: about 4.5% a year, guaranteed and tax-free, which is exactly your mortgage rate.
How Does Your Interest Rate Change the Answer?
The interest rate is the single biggest driver of what an overpayment is worth. The years cut barely move with the rate, but the pounds saved swing enormously, because a higher rate means more interest to remove. Same £200,000 loan over 25 years:
| Mortgage rate | Standard payment | £100/month saves | £200/month saves | £500/month saves |
|---|---|---|---|---|
| 3.5% | £1,001 | £15,100 (21.7 yrs) | £26,200 (19.1 yrs) | £46,800 (14.1 yrs) |
| 4.5% | £1,112 | £21,100 (21.5 yrs) | £36,300 (18.9 yrs) | £63,900 (14.0 yrs) |
| 5.5% | £1,228 | £28,100 (21.4 yrs) | £47,800 (18.8 yrs) | £82,800 (13.8 yrs) |
| 6.5% | £1,350 | £36,200 (21.3 yrs) | £60,800 (18.6 yrs) | £103,500 (13.7 yrs) |
At 6.5%, £200 a month saves more than £500 a month does at 3.5%. This is why the overpay-or-invest debate flips with the rate cycle: on a 1.5% fix from 2021 overpaying was hard to justify against an ISA, while on a 5.5% remortgage in 2024 it became a very respectable guaranteed return. If you are coming off a cheap fix onto a higher rate, an overpayment you dismissed three years ago may now be the best low-risk use of spare cash you have.
Does the Size of Your Mortgage Matter?
Less than you might think for the pounds saved, but a great deal for the years cut. A fixed £200 a month is a much bigger proportion of a £150,000 mortgage payment than of a £400,000 one, so it shortens the smaller loan far more. All at 4.5% over 25 years:
| Mortgage balance | Standard payment | £100/month | £200/month | £500/month |
|---|---|---|---|---|
| £150,000 | £834 | £20,000 saved, 20.6 yrs | £33,200 saved, 17.5 yrs | £54,900 saved, 12.3 yrs |
| £200,000 | £1,112 | £21,100 saved, 21.5 yrs | £36,300 saved, 18.9 yrs | £63,900 saved, 14.0 yrs |
| £300,000 | £1,667 | £22,400 saved, 22.6 yrs | £40,100 saved, 20.6 yrs | £76,400 saved, 16.3 yrs |
| £400,000 | £2,223 | £23,100 saved, 23.2 yrs | £42,300 saved, 21.5 yrs | £84,700 saved, 17.8 yrs |
£500 a month on a £150,000 mortgage is a 60% overpayment that clears the loan in a little over 12 years. The same £500 on a £400,000 London mortgage is a 22% overpayment and still leaves nearly 18 years to run. If you have a large mortgage and a FIRE date in mind, the useful mental model is overpayment as a percentage of your standard payment, not a round number of pounds: an 18–20% overpayment cuts roughly six years from a 25-year term whatever the balance, and a 45% overpayment cuts roughly eleven.
Why Starting Early Matters So Much
Every figure above assumes you overpay from the first month. Interest on a repayment mortgage is front-loaded: in year one of the £200,000 example, about £8,900 of your £13,300 in payments is interest and only £4,400 is capital. By year 20 those proportions have reversed. So an overpayment early in the term removes a pound of balance that would otherwise have been charged interest for two decades; the same pound in year 20 is charged interest for five years.
If you start the same overpayments in year 10 instead of year one, with 15 years and roughly £145,000 left:
- £100 a month saves about £6,700 and cuts 1.7 years, against £21,100 and 3.5 years from day one.
- £200 a month saves about £11,900 and cuts 3.0 years, against £36,300 and 6.1 years.
- £500 a month saves about £22,400 and cuts 5.7 years, against £63,900 and 11 years.
The late starter saves roughly a third as much interest. That is not a reason to skip overpaying in the second half of a mortgage, because the return on each pound is still your mortgage rate, tax-free. It is a reason not to wait for a “better time” if you have decided overpaying is right for you. A modest £100 from the beginning beats a heroic £500 started in year 15.
The Setting That Can Halve Your Saving: Reduce the Term, Not the Payment
When you set up an overpayment, most UK lenders will ask, or quietly assume, whether you want to reduce the term or reduce the monthly payment. Everything above assumes the term is reduced: your payment stays at £1,112 plus the overpayment, and the mortgage finishes early. If the lender instead recalculates your standard payment downwards each time you overpay, your mortgage still runs the full 25 years and your “overpayment” slowly becomes just your normal payment.
Rerunning £200 a month on the £200,000 example with the payment recalculated rather than the term shortened, the interest saved falls from £36,300 to about £19,000, and you are still paying the mortgage in year 25. You have handed back nearly half the benefit for the privilege of a slightly smaller direct debit. Reducing the payment can make sense if you want flexibility, for example if your income is volatile and you might need the lower payment later, but it should be a deliberate choice. Check your lender’s app or ring them: most of the big lenders let you choose, and several default to reducing the payment unless you say otherwise.
Also check your overpayment sits inside your penalty-free allowance. £500 a month is £6,000 a year, only 3% of a £200,000 loan and comfortably under the 10% most deals allow. Our guide to the 10% overpayment rule covers how lenders measure it, and our early repayment charge guide explains what happens if you go over.
Should You Overpay or Invest the £200 Instead?
Interest saved is the wrong yardstick for this question, because the alternative is not “do nothing” but “invest the money”. The fair comparison is total wealth at the end of the 25 years. Two routes for the same £200 a month:
- Invest all along: £200 a month into a global index fund inside a Stocks & Shares ISA for 25 years, keeping the mortgage on schedule.
- Overpay then invest: £200 a month overpaid until the mortgage clears at 18.9 years, then the full £1,312 a month (freed payment plus overpayment) invested for the remaining 6.1 years.
| Investment return | Invest all along (25 years) | Overpay, then invest | Winner |
|---|---|---|---|
| 5% a year | £119,100 | £111,600 | Roughly level |
| 7% a year | £162,000 | £118,900 | Investing, by £43,000 |
Both routes end mortgage-free at year 25 with a pot of money, so the difference is purely the return. At a 5% investment return against a 4.5% mortgage, the two are within a few thousand pounds and the guaranteed route is arguably better. At 7%, close to the long-run average for global equities, investing pulls ahead by more than £40,000, but only by accepting 25 years of market risk. Tax tilts it further towards investing: ISA returns are tax-free, and if the alternative is a salary-sacrifice pension with 40% relief, overpaying rarely competes. Our pay off the mortgage or invest guide works through this in depth, and our overpayments and your FIRE date guide shows the effect on your independence date rather than your mortgage date.
The practical answer most UK FIRE pursuers land on is a split: keep the pension match and fill the ISA first, then overpay with whatever is left, and lean harder into overpaying as the mortgage rate rises or the FIRE date gets close. Being mortgage-free cuts the income you need in retirement, which cuts your FIRE number by 25 times your annual mortgage payment, and that certainty is worth a lot in the final few years. The MoneyHelper guide to overpaying is a good independent checklist before you commit.
Frequently Asked Questions
What happens if I overpay my mortgage by £200 a month?
On a £200,000 mortgage at 4.5% over 25 years, overpaying £200 a month from the start clears the loan in 18.9 years instead of 25, cutting just over six years from the term, and saves about £36,300 in interest. You pay in roughly £45,400 of overpayments over those 18.9 years, so each £1 overpaid removes about 80p of interest. That assumes your lender keeps the monthly payment the same and shortens the term. If the lender recalculates your payment downwards instead, the same £200 a month saves only about £19,000.
Is it worth overpaying a mortgage by £100 a month?
Yes, if you have no expensive debt, an emergency fund and are already getting your full employer pension match. £100 a month on a £200,000 mortgage at 4.5% saves about £21,100 of interest and clears the loan 3.5 years early, for a total outlay of £25,800. The return is a guaranteed, tax-free 4.5% on every pound, which beats most savings accounts after tax. It is likely to be worth less than investing the same £100 in a Stocks and Shares ISA if markets return their long-run average, so the choice depends on your risk appetite and how close you are to FIRE.
How many years does overpaying £500 a month take off a mortgage?
On a £200,000 mortgage at 4.5% over 25 years, £500 a month clears the loan in 14.0 years, taking 11 years off the term and saving about £63,900 of interest. On a £150,000 mortgage at the same rate it clears in 12.3 years and saves £54,900; on a £300,000 mortgage it clears in 16.3 years and saves £76,400. £500 a month is £6,000 a year, well inside a typical 10% annual overpayment allowance, so it will not trigger an early repayment charge on most deals.
Does overpaying reduce the term or the monthly payment?
It depends on what you tell the lender. Many UK lenders default to reducing your monthly payment and keeping the term unchanged, which hands back most of the benefit. On a £200,000 mortgage at 4.5%, £200 a month with the term shortened saves about £36,300 and clears the loan six years early; the same £200 with the payment reduced instead saves only about £19,000 and the mortgage still runs the full 25 years. Always ask for the term to be reduced, or check the option in your online banking, when you set up an overpayment.
Is it better to overpay the mortgage or invest the money?
Overpaying gives a guaranteed, tax-free return equal to your mortgage rate. Investing £200 a month in a global index fund inside an ISA at a 7% average return for 25 years would grow to about £162,000, versus £119,000 if you overpaid first and then invested the freed-up payment. At a 5% return the two approaches finish almost level. Overpaying wins on certainty and peace of mind; investing wins on expected wealth if your mortgage rate is below about 5% and you can tolerate market swings. Many UK FIRE pursuers do both: fill the ISA first, then overpay with what is left.
Work Out Your Own Numbers
Your rate, balance and remaining term will not match the examples exactly, so run your own:
- Mortgage Overpayment Calculator — enter your balance, rate and term, then try £100, £200 and £500 a month to see the interest saved and the new mortgage-free date
- Principal Payback Calculator — see how much of each payment is capital versus interest, and what doubling the capital portion does to the term
See What Overpaying Does to Your FIRE Date
FIRE Finance tracks your mortgage, overpayments, ISAs and pensions in one place, so you can see whether £200 a month is better on the mortgage or in the market for your own plan — built for the UK.
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