Loan Overpayment Calculator — But Should You Overpay At All?

Your normal monthly payment is split: part interest, part capital. Early in a mortgage that split is brutal — most of what you pay is interest, and the balance barely moves.

An overpayment is different. Every extra pound goes straight at the capital. And because interest is charged on the balance, killing a pound of capital today kills every pound of interest that pound would have generated for the rest of the term.

That's why the numbers look almost implausible. £200 a month on a £180,000 mortgage at 4.5% with 22 years left doesn't save you £200 × 12 × 22. It saves you far more, because you're not just paying down debt — you're deleting decades of compounding.

It also means timing matters enormously. The same £10,000 overpaid in year 2 saves several times what it saves in year 18. If you're going to do this, earlier is dramatically better.

This is the single most expensive thing people get wrong about overpaying, and it happens quietly, by default.

When you overpay, the lender can do one of two things: shorten the term — your payment stays the same, the balance falls faster, and you finish years early, which is what you want — or reduce your monthly payment, where the balance still falls, but the lender recalculates your payment downwards so you finish on the original date. You've handed them your money and bought yourself a slightly cheaper month — while still paying interest for the full term.

Same overpayment. Wildly different outcome. Reducing the term saves several times more interest.

Many lenders default to reducing the payment unless you tell them otherwise. Tell them otherwise, in writing. The calculator above lets you toggle between the two so you can see the size of the gap for yourself — and then go and check what your lender is actually doing with the money you've already sent them.

Most UK fixed-rate mortgages let you overpay up to 10% of the outstanding balance each year with no penalty. On a £200,000 balance that's £20,000 a year — roughly £1,667 a month, which is far more than most people are overpaying anyway.

Two details worth knowing. The allowance is normally recalculated each year against the falling balance, so it shrinks as you pay down. And it typically resets on your product anniversary, not on 1 January — though this varies by lender. Halifax, for instance, calculates the allowance against the balance owed as of 1 January, while HSBC resets it on the date the account was opened, or the start date of your current fixed rate if you've switched. This matters if you're planning a big lump sum around a bonus — check your specific lender's method before assuming.

Breach it and the lender charges an early repayment charge, usually 1%–5%, often stepping down each year of the deal. Some lenders apply it only to the excess above the allowance; others apply it to the whole overpayment. That difference is enormous — on a £20,000 allowance with a £25,000 overpayment, a 3% ERC on just the £5,000 excess is £150, but the same rate on the whole £25,000 is £750. Find out which yours does before you send anything.

If you're on a tracker or the SVR, there's usually no limit at all. Tick the box in the calculator and the ERC check switches off.

A tactic worth knowing: if you've got a big lump sum and the allowance is in the way, split it. Pay part before your product anniversary and part after, and you get two years' allowance out of one windfall. Or simply wait until the fix ends — ERCs vanish once you're on the SVR.

The ranker mode above runs this against your actual numbers and tells you where your next £1 should go.

"My savings account pays 4.4% and my mortgage is 4.5%, so it's basically a wash." No, it isn't — and the reason is tax.

Overpaying saves you interest. Saved interest is not income, so it isn't taxed. A 4.5% mortgage overpayment is a guaranteed, tax-free 4.5%.

Savings interest is income. Once you've used your Personal Savings Allowance (£1,000 basic rate, £500 higher rate, £0 additional rate), you pay your marginal rate on it: 4.4% savings → 3.52% net (basic) · 2.64% net (higher) · 2.42% net (additional).

So for a higher-rate taxpayer, a 4.5% mortgage doesn't narrowly beat a 4.4% savings account — it demolishes it, by nearly two full percentage points a year.

A cash ISA is different: it's tax-free, so you compare it directly against your mortgage rate. If your ISA pays 4.2% and your mortgage costs 4.5%, overpaying still wins — but it's a fair fight, and the ISA keeps your money accessible, which has real value.

Note the exception: an offset mortgage gives you both. Your savings sit in a linked account, reduce the interest charged, and stay withdrawable. If you have one, use it.

The honest verdict: take the employer match first, always. After that it's a genuine trade-off between mathematical return and the psychological and practical value of being debt-free. The pension wins on paper. Plenty of people rationally choose the mortgage anyway — the money is locked until 57, and a pension doesn't help you sleep at night when rates are climbing.

What you should not do is overpay while ignoring a match. That's not a trade-off; that's just leaving money on the table.

You have no emergency fund. Overpayments are one-way. You cannot ring the lender and ask for it back because the car died. Three to six months of expenses in an accessible account comes first, every time — and it isn't close.

You're carrying expensive debt. A credit card at 22.9% costs you five times what your mortgage does. Clearing it is a guaranteed 22.9% return. Overpay the card, not the house.

You're declining an employer pension match. Free money. Take it.

You might move soon. Overpaying builds equity, which is fine — but if you're selling in 18 months, the interest saving is small and you've locked cash into bricks you're about to sell anyway.

Your ERC would eat the benefit. If a lump sum breaches the allowance and triggers a 3% charge, you may have paid £3,000 to save £2,000. The calculator flags this.

None of this means don't overpay. It means overpay in the right order — and once you get there, it's one of the best risk-free returns available to a UK household.

The arithmetic is identical for any amortising loan — just enter the balance, rate and remaining term. What changes is the penalty regime, and it's usually far friendlier than a mortgage.

Under the Consumer Credit Act, if you settle a regulated personal loan early, the lender can generally charge no more than about 58 days' interest. That's a fraction of a mortgage ERC. On most personal loans, early repayment is close to free.

Car finance is different. On PCP, the balloon payment and the way interest is front-loaded mean overpaying doesn't always work the way you'd expect. Ask for a settlement figure in writing before you assume anything.

And the obvious point that bears repeating: if you're carrying a personal loan at 8% and a mortgage at 4.5%, the loan gets the money. Every time.

You owe £180,000 at 4.5% with 22 years left. Your payment is about £1,120 a month. You've got £200 spare.

Overpay £200/month, shortening the term

You clear the mortgage roughly 4½ years early and save somewhere around £25,000 in interest. From £200 a month. That's the compounding working in your favour instead of against you.

Now let the lender reduce your payment instead

Same £200 a month. Your monthly payment creeps down, you finish on the same date, and the interest saving collapses to a fraction of that. Same money, dramatically worse outcome — and it happens by default at plenty of lenders.

Check the ERC

£200 × 12 = £2,400 a year. Your 10% allowance is £18,000. You're using 13% of your headroom. No problem at all — you could overpay six times as much before the charge bites.

But should you?

You're a higher-rate taxpayer with a £3,000 credit card at 22.9%, two months of emergency fund, and an employer who matches pension contributions.

So no — not yet. That £200 should go: emergency fund (to 3 months), then the credit card (a guaranteed 22.9%, five times your mortgage rate), then the pension match (an instant 100%, plus 40% relief). Then the mortgage.

The overpayment was never a bad idea. It was just fourth in the queue — and every calculator that showed you the £25,000 without mentioning the credit card did you a disservice.

Official sources & further reading: check the Personal Savings Allowance on GOV.UK, read pension tax relief rules, and see the overpayment guidance at MoneyHelper. Browse every tool on the mortgage calculators homepage.