Mortgage Overpayment Calculator UK
See exactly how much a regular or one-off overpayment saves in interest, how many years it cuts from your term, and whether overpaying beats saving or investing the same money.
Overpaying a mortgage is the closest thing a homeowner has to a guaranteed, tax-free return. Every extra pound goes straight at the capital, wiping out all the future interest that pound would have generated. On a £250,000 mortgage at 5%, overpaying just £200 a month saves around £44,000 in interest and clears the loan nearly five years early. But the timing matters enormously — the same lump sum saves far more in year one than in year fifteen — and there’s a 10% annual limit on most fixed deals before an early repayment charge bites. This calculator shows your exact saving, and the big question it can’t answer alone: with spare cash, is overpaying smarter than an ISA, a pension, or simply keeping it? To work out your base monthly payment first, use the Mortgage Calculator.
Mortgage details
Overpayment plan
Allowance and early repayment charges
Comparison assumptions
Overpayment result
Interest saved
Calculating…
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Time saved
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New payoff time
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Current payment
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Total overpaid
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ERC estimate
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Net saving after ERC
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What overpaying saves — quick lookup
Two ready-reckoners for a £250,000 mortgage at 5% over 25 years (base payment £1,461/month, total interest £188,443 with no overpayments). The left shows a regular monthly overpayment; the right shows a single lump sum paid early. Both show the interest saved and the new term. The calculator above runs your exact figures.
| Extra/mo | Interest saved | New term |
|---|---|---|
| +£50 | £13,676 | 23.5yr |
| +£100 | £25,382 | 22.1yr |
| +£200 | £44,432 | 19.8yr |
| +£300 | £59,322 | 18.0yr |
| +£500 | £81,204 | 15.3yr |
| Lump sum | Interest saved | New term |
|---|---|---|
| £5,000 | £12,023 | 24.1yr |
| £10,000 | £23,260 | 23.2yr |
| £25,000 | £52,904 | 20.6yr |
Look at the lump-sum column closely. A £10,000 overpayment saves £23,260 in interest — more than double the cash put in. That’s the power of removing capital early: you don’t just save the £10,000, you erase 23 years of compounding interest on it. The same £10,000 paid in year fifteen saves only around £6,000, because by then most of the interest has already been charged. Timing is everything.
How mortgage overpayments work
A mortgage charges interest on the outstanding balance every month. When you make your normal payment, part covers that month’s interest and part reduces the balance (the capital). An overpayment is pure capital — it skips the interest split entirely and goes straight at what you owe. That immediately reduces the balance the next month’s interest is charged on, which is why a single overpayment keeps saving you money for the rest of the term.
Why early overpayments are worth so much more
This is the part most people miss. A mortgage is front-loaded: in the early years, the balance is huge, so the interest charged each month is huge. Repay £10,000 of capital in year one and you remove interest on that £10,000 for the remaining 24 years. Repay the same £10,000 in year fifteen and there are only ten years of interest left to remove — and a smaller balance was generating it anyway. The result is stark: that £10,000 saves £23,260 if paid in year one but only about £6,000 in year fifteen. Same money, nearly four times the benefit, purely because of when it lands.
Reduce the term, or reduce the payment?
When you overpay, most UK lenders let you choose what happens: shorten the term (keep paying the same monthly amount, finish years early) or reduce future payments (keep the same end date, pay less each month). Shortening the term saves dramatically more interest, because you’re keeping the money working against the balance. Reducing the payment gives you breathing room now but throws away most of the benefit. Unless you genuinely need lower monthly outgoings, choose to shorten the term — and check your lender does it automatically, as some default to reducing the payment instead.
The 10% rule and early repayment charges
Here’s the catch that trips people up. Most UK fixed-rate deals cap penalty-free overpayments at 10% of the outstanding balance per year. Pay in more than that during the fixed period — a large inheritance or bonus, say — and you’ll usually trigger an early repayment charge (ERC) of 1–5% of the amount above the limit. On a £250,000 balance, the 10% allowance is £25,000 a year, which is plenty for most people. But if you’re planning a big lump sum, check your allowance first, and consider timing it for just after a deal anniversary (the allowance resets) or once the fixed period ends, when ERCs usually disappear. The FCA requires lenders to set out your ERC terms clearly in your mortgage illustration.
Four worked examples
Real overpayment outcomes on the same £250,000 mortgage at 5%, showing how the type and timing of an overpayment change the result.
Example 1 — Sam (the steady £100)
£100 a month, every month, from day one
With +£100/mo: pays £1,561/mo
Interest saved: £25,382 · Term: 22.1 years
Sam rounds his payment up by £100 a month — barely noticeable in the budget, the price of a few takeaways. Over the life of the mortgage that small habit saves over £25,000 in interest and clears the loan nearly three years early. The lesson: overpaying doesn’t need to be dramatic. Modest, consistent amounts compound into serious savings because every one of them removes capital early.
Example 2 — Nadia (the bonus lump sum)
£10,000 work bonus, paid in early
Interest saved: £23,260 · Term: 23.2 years
Return on the £10,000: effectively 5% tax-free, compounding
Nadia puts her annual bonus straight onto the mortgage. The £10,000 saves her £23,260 — she more than doubles her money in interest avoided. Crucially, she checks her 10% annual allowance first (£25,000 on her balance), so there’s no ERC. Had she sat the bonus in a regular savings account at 4%, she’d have earned a few hundred pounds of taxable interest instead. Against a 5% mortgage, overpaying won easily.
Example 3 — Tom (left it too late)
£10,000 lump sum — but in year 15
Interest saved: £6,129
Same £10k in year 1 would have saved: £23,260
Tom does exactly what Nadia did, with the same £10,000 — but waits until year fifteen. His saving is just £6,129, less than a third of what an early overpayment achieves. By year fifteen, the bulk of the interest on his original loan has already been charged; there’s simply less left to remove. It’s not that late overpayments are pointless, but they’re a fraction as powerful. If you’re going to overpay, the worst day to start is tomorrow’s tomorrow.
Example 4 — Grace (the ERC trap avoided)
£40,000 inheritance, mid-fixed-period
Overpay £25,000 now (within allowance): no charge
Overpay full £40,000: ERC on £15,000 excess at ~3% = £450
Grace inherits £40,000 and wants it all on the mortgage. But she’s two years into a five-year fix, with a £25,000 annual overpayment allowance. Paying the full £40,000 would trigger an ERC on the £15,000 excess. Her smarter move: overpay £25,000 now, hold the remaining £15,000 in a high-interest account, and overpay it after her next deal anniversary when the allowance resets. She gets nearly all the benefit and dodges the penalty entirely.
Overpay, or do something else with the money?
This is the real question behind every overpayment, and the one most calculators ignore. You’ve got spare cash — should it go on the mortgage, into an ISA, into a pension, or stay as savings? There’s no single right answer, but there is a clear order of priorities for most UK households. Here’s how the options stack up, roughly in the order they usually make sense:
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1
Clear expensive debt first
Before overpaying a mortgage, clear anything charging more than your mortgage rate — credit cards (often 20%+), car finance, personal loans. Overpaying a 5% mortgage while carrying 22% credit card debt is moving money the wrong way.
The maths is simple: pay down the highest interest rate first, always. A mortgage is usually the cheapest debt you’ll ever have.
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2
Capture the pension match
If your employer matches pension contributions, that’s an instant 100% return plus tax relief — nothing else comes close. A higher-rate taxpayer gets 40% tax relief on top of any match.
For most people, maxing the employer match beats overpaying the mortgage by a wide margin, even though pension money is locked away until later life.
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3
Keep an emergency fund
Overpayments are usually locked into the house — you can’t easily get the money back if you lose your job or face a big bill. Hold 3–6 months of essential outgoings in an easy-access account before overpaying.
A paid-down mortgage is no help in a cash emergency. Liquidity has value the interest saving doesn’t capture.
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Then: overpay vs invest
With debt cleared, match captured, and a buffer in place, the choice is overpay or invest. Overpaying is a guaranteed, tax-free return equal to your mortgage rate. Investing in a stocks & shares ISA might beat it over the long run, but it isn’t guaranteed.
£200/month overpaying saves a guaranteed £44,000. The same invested might grow to £82,000 over 20 years at 5% — but with risk. Overpaying suits the cautious; investing suits the patient.
£200/month for 20 years — where it lands
Same £48,000 of contributions, different homes:
The investment figures look bigger, and over a 20-year horizon they often win — historically, diversified equity returns have beaten typical mortgage rates. But “often” isn’t “always”, and the overpayment return is locked in regardless of what markets do. The honest answer is that overpaying versus investing is partly a maths question and partly a temperament question: some people sleep better with a smaller mortgage, and that peace of mind is worth something the spreadsheet can’t show.
The guaranteed-return framing
The cleanest way to think about overpaying: it earns you a guaranteed, tax-free return equal to your mortgage rate. Overpay a 5% mortgage and you’ve effectively earned 5%, risk-free and untaxed — which is genuinely hard to beat in a savings account, especially for a higher-rate taxpayer who’d lose 40% of any interest earned elsewhere. Compare that 5% guaranteed against what you could earn (after tax) on cash savings, and overpaying usually wins on the cash side. It’s only the riskier, longer-term investing comparison where the answer gets interesting.
Two scenarios worth modelling
What if…
You overpaid £300 instead of £100?
What if…
You overpaid past the 10% limit?
Key overpayment terms explained
Overpaying has its own small vocabulary, and the difference between two of these terms can cost you a four-figure penalty. The ten below cover what you’ll meet when you call your lender to overpay.
- Overpayment
- Any payment beyond your required monthly amount. It goes entirely to capital, reducing the balance interest is charged on. Can be regular (a fixed extra each month) or a one-off lump sum. The single most effective way to cut the total cost of a mortgage.
- Capital
- The amount you still owe — the outstanding balance, separate from interest. Overpayments reduce capital directly, which is why they’re so powerful: less capital means less interest charged every month thereafter, for the rest of the term.
- Early repayment charge ERC
- A penalty for overpaying beyond your annual allowance or clearing the mortgage during a fixed period — typically 1–5% of the amount above the limit, tapering each year of the deal. The main reason to check your allowance before paying in a large lump sum.
- 10% annual allowance
- The amount most UK fixed deals let you overpay penalty-free each year — usually 10% of the outstanding balance. On a £250,000 balance that’s £25,000 a year, plenty for most. The allowance typically resets on the deal’s anniversary.
- Reduce term vs reduce payment
- The choice when you overpay: keep paying the same and finish earlier (reduce term, saves most interest), or keep the same end date and pay less monthly (reduce payment, throws away most of the benefit). Choose reduce-term unless you need lower outgoings now.
- Front-loading
- The way mortgage interest is concentrated in the early years, when the balance is largest. It’s why early overpayments save far more than late ones — a pound repaid in year one erases interest for the whole remaining term.
- Offset mortgage
- A mortgage linked to a savings account, where your savings balance is “offset” against the loan, reducing the interest charged without permanently committing the cash. An alternative to overpaying that keeps your money accessible — useful if you want the interest saving but not the loss of liquidity.
- Effective return
- The way to compare overpaying with saving or investing. Overpaying earns a guaranteed, tax-free return equal to your mortgage rate. Overpay a 5% mortgage and you’ve effectively earned 5% risk-free — hard to match in a taxed savings account.
- Emergency fund
- Easy-access cash covering 3–6 months of essential outgoings. Should come before overpaying, because overpayments are locked into the property — a paid-down mortgage doesn’t help in a cash emergency. Liquidity has a value the interest saving doesn’t capture.
- Pension match
- Employer pension contributions matched to yours — an instant 100% return plus tax relief. For most people this beats overpaying a mortgage, even though pension money is locked away until later life. Capture the full match before overpaying.
Five mistakes UK homeowners make overpaying
Overpaying is almost always smart, but these errors waste some of the benefit — or, in two cases, actively cost money. All come from broker experience and the recurring r/UKPersonalFinance threads.
Overpaying past the 10% limit during a fix
A windfall paid straight onto the mortgage feels right, but most fixes cap penalty-free overpayments at 10% of the balance a year. Exceed it and you’ll pay an ERC of 1–5% on the excess. Split a large lump across deal anniversaries, or wait for the fix to end — the allowance resets each year and ERCs usually vanish at the end of the deal.
Cost: hundreds to thousands in ERC Fix: check your allowance, split large sumsChoosing “reduce payment” instead of “reduce term”
When you overpay, the lender asks whether to lower your future payments or shorten the term. Reducing the payment feels nice but throws away most of the interest saving, because the balance stays high for longer. Some lenders even default to it silently. Unless you genuinely need lower monthly outgoings, always choose to shorten the term.
Cost: most of the interest saving lost Fix: tell your lender to reduce the termOverpaying while carrying expensive debt
Overpaying a 5% mortgage while holding a credit card at 22% or car finance at 12% is moving money the wrong way. Clear the highest-rate debt first, always. A mortgage is usually the cheapest borrowing you’ll ever have — the last debt to throw spare cash at, not the first.
Cost: paying 22% to save 5% Fix: clear expensive debt before overpayingEmptying the emergency fund to overpay
Overpayments are locked into the house — you can’t easily pull them back if you lose your job or face a sudden bill. Tipping your entire savings buffer onto the mortgage leaves you asset-rich but cash-poor, and forced to borrow expensively in a crisis. Hold 3–6 months of essential outgoings in easy-access savings first.
Cost: forced expensive borrowing in a crisis Fix: keep 3–6 months expenses accessibleSkipping the employer pension match
An employer pension match is an instant 100% return plus tax relief — no overpayment comes close. Yet many people overpay the mortgage while leaving free pension money on the table. Capture the full match first, then overpay. The mortgage saving is excellent, but a guaranteed doubling of your money beats it comfortably.
Cost: free 100% return left unclaimed Fix: max the pension match, then overpayFrequently asked questions
How much does overpaying my mortgage actually save?
It depends on the amount, your rate, and your remaining term — but the savings are substantial. On a £250,000 mortgage at 5% over 25 years, overpaying £100 a month saves around £25,000 in interest and clears the loan nearly three years early. Overpay £200 a month and you save about £44,000.
A one-off lump sum is just as powerful when paid early: £10,000 overpaid in year one saves £23,260 — more than double the cash put in. The calculator above shows your exact figures.
Is it better to overpay or save the money instead?
For cash savings, overpaying usually wins. Overpaying earns a guaranteed, tax-free return equal to your mortgage rate — overpay a 5% mortgage and you’ve effectively earned 5%, risk-free and untaxed. A savings account would need to pay well above 5% (before tax) to beat that, especially for a higher-rate taxpayer.
The closer call is overpaying versus investing in a stocks & shares ISA, which might grow faster over the long term but isn’t guaranteed. First, though, clear expensive debt, capture any employer pension match, and keep an emergency fund — those come before either option.
How much can I overpay without a penalty?
Most UK fixed-rate deals let you overpay up to 10% of the outstanding balance per year with no penalty. On a £250,000 balance, that’s £25,000 a year — more than enough for most people. The allowance usually resets on your deal’s anniversary.
Go beyond the 10% limit during a fixed period and you’ll typically trigger an early repayment charge of 1–5% on the excess. If you’re planning a large lump sum, check your specific allowance first, and consider splitting it across deal anniversaries or waiting until the fix ends.
Should I reduce my term or reduce my monthly payment?
To save the most interest, choose to reduce the term — keep paying the same monthly amount and finish years early. This keeps your money working against the balance for maximum effect.
Reducing the payment instead (same end date, lower monthly cost) gives you breathing room now but throws away most of the interest saving, because the balance stays higher for longer. Some lenders default to reducing the payment unless you tell them otherwise — so be explicit. Only choose reduce-payment if you genuinely need lower outgoings.
Does it matter when I make an overpayment?
Enormously. Mortgages are front-loaded — the balance is largest in the early years, so the interest charged is largest then too. An overpayment made early removes interest for the whole remaining term; one made late removes far less.
The same £10,000 saves around £23,260 if paid in year one, but only about £6,000 in year fifteen. If you’re going to overpay, do it as early as you can — every year you delay reduces the benefit. Regular monthly overpayments capture this advantage automatically by starting straight away.
Should I overpay or pay into my pension?
If your employer matches pension contributions, capture the full match first — it’s an instant 100% return plus tax relief, which no mortgage overpayment can match. A higher-rate taxpayer also gets 40% tax relief on pension contributions.
Beyond the match, it’s a trade-off. Pension money grows tax-advantaged but is locked away until later life; overpaying frees you from debt sooner and is fully accessible as home equity (if not as cash). Many people do both — max the match, then split spare cash between overpaying and a pension or ISA.
Can I get overpaid money back if I need it?
Usually not easily. Standard overpayments are locked into the property — they reduce your balance, but you can’t simply withdraw them. Some lenders offer “borrow back” or “overpayment reserve” features that let you redraw, but these aren’t universal, so check before relying on it.
This is why an emergency fund matters more than overpaying for most people: a paid-down mortgage doesn’t help in a cash crisis. If keeping access to your money matters, an offset mortgage — which links savings to the loan without locking them in — may suit you better than overpaying.
Will overpaying affect my credit score?
No — overpaying has no negative effect on your credit score, and clearing debt faster is generally viewed positively. You’re simply reducing what you owe ahead of schedule.
The only thing to watch isn’t credit-related: avoid triggering an early repayment charge by overpaying beyond your annual allowance. That’s a cost, not a credit issue. For impartial guidance on managing mortgage payments, MoneyHelper is a good independent source.
Related calculators
Overpaying is one financial decision among several. These calculators handle the cost it reduces, and the alternatives it competes with.
Methodology & sources
How the maths works
The calculator models a repayment mortgage month by month. Each month, interest is charged on the outstanding balance and the regular payment reduces it; an overpayment is applied directly to the capital, immediately lowering the balance the next month’s interest is calculated on. For a regular monthly overpayment, the extra is added every month until the balance reaches zero. For a one-off lump sum, the balance is reduced at the chosen point and the remaining schedule recalculated.
Interest saved is the difference between the total interest on the original schedule and the total interest with overpayments. The new term is the point at which the overpaid balance reaches zero. The early-versus-late difference arises naturally from the month-by-month model: a lump sum applied early removes interest across more remaining months than the same sum applied late.
UK rules and conventions used
- Overpayment: applied entirely to capital, monthly compounding
- 10% annual allowance: typical penalty-free overpayment limit on fixed deals
- Early repayment charge: typically 1–5% of the excess above the allowance
- Reduce term: assumed default in examples (saves most interest)
- Effective return: overpaying = guaranteed tax-free return at the mortgage rate
- Rates shown are illustrative to demonstrate the maths, not live market rates