What this calculator works out
Buying a home in the UK involves two numbers that arrive at very different moments, and most tools only give you one of them. The first is what you pay every month for the next few decades. The second is the lump sum you have to find on completion day, which is the deposit plus Stamp Duty and is often the thing that decides whether an offer is realistic at all.
This calculator produces both, alongside the interest-only cost for comparison, a payment-by-payment schedule, and the effect of overpaying. The figures update as you type, so you can drag a deposit percentage or a term around and watch what actually moves.
The formula behind the monthly figure
A capital repayment mortgage uses the standard annuity formula, in which L is the sum you borrow, r is one twelfth of the annual rate, and n is how many monthly payments the term contains:
That single payment is then walked forward one month at a time, which is what turns a number into a schedule:
Capital this month = payment − interest this month
New balance = balance − capital this month
Everything else follows from that loop. Total interest is the sum of the interest column, and the final payment lands in the month the balance reaches zero. Interest-only is far simpler: the payment is just L × r, the balance never moves, and the whole debt is still outstanding at the end.
Running costs sit outside the loan entirely. Council tax, insurance, protection cover and anything else you enter are annual amounts divided by twelve for the monthly column and multiplied across the term for the lifetime column. They change what leaving the house costs you; they change nothing about the mortgage.
What each field means
- Property price and deposit. The deposit is a percentage, so the loan is the price less that share. The remainder is your loan-to-value ratio, and it is the single biggest lever on the rate you will be offered — the gap between a 90% and a 75% LTV deal is usually much wider than anything you can negotiate on fees.
- Mortgage term. Twenty-five years was the long-standing default; longer terms have become common as prices outpaced wages. Stretching the term thins out each payment while thickening the lifetime bill.
- Interest rate. The rate on your product, not the APRC. APRC folds in fees and the assumed reversion rate, so it reads higher than the rate your payments are actually built on.
- Buying as. This drives the Stamp Duty figure only. First-time buyer, home mover and additional property are taxed on three different scales.
- Overpayment. A voluntary amount added to every payment and applied straight to the balance. The schedule rebuilds around it, so the final payment moves in and the interest total drops.
- First payment. Sets the dates on every row and the month the mortgage ends. It changes no monetary figure.
- Running costs. Council tax is entered as a pound amount a year rather than a percentage of the price, because that is how it works — it is set by band and by council, and a band D bill in one authority can be hundreds of pounds away from the same band elsewhere.
Repayment or interest-only
The calculator shows both figures side by side deliberately, because the gap between them is the clearest illustration of what you are actually buying. On £289,000 at 5.24%, the repayment figure is £1,594 a month and the interest-only figure is £1,262. The £332 difference is the only part of that first payment that reduces what you owe.
That is worth sitting with. In month one of a thirty-year mortgage, roughly four fifths of the payment is rent on the money and one fifth is ownership. The ratio flips slowly, and the crossover point — where more of the payment goes to capital than to interest — does not arrive until month 202 of 360, a little past the halfway mark. It is the main reason equity feels immovable in the early years.
Stamp Duty, band by band
Stamp Duty Land Tax applies in England and Northern Ireland and is charged in slices, exactly like income tax. A common misreading is to see a price sitting in the 5% band and assume 5% of the whole purchase is due. It is not: each band taxes only the portion of the price that falls inside it.
For completions from 1 April 2025 the standard scale runs at nothing up to £125,000, 2% to £250,000, 5% to £925,000, 10% to £1.5 million and 12% above that. First-time buyers pay nothing to £300,000 and 5% between £300,001 and £500,000, with the relief withdrawn entirely — not tapered — once the price passes £500,000. Anyone who will own more than one residential property after the purchase pays a further 5% on every slice.
The breakdown table on this page shows the arithmetic rather than just the answer, so you can see which slice each pound of duty came from. If you are buying in Scotland or Wales the figure will not apply to you: those nations charge Land and Buildings Transaction Tax and Land Transaction Tax respectively, on their own thresholds.
What overpaying actually saves
Overpaying is the highest-return move available to most borrowers, and the reason is compounding in reverse. An extra payment does not simply reduce the balance once; it removes that sum from every future interest calculation for the rest of the term.
On the default scenario — £289,000 over 30 years at 5.24% — adding £150 a month saves roughly £59,000 in interest and clears the mortgage five years and four months early. The £150 never earns a stated return anywhere, yet it displaces interest at the mortgage rate, tax-free, with certainty.
Two practical limits are worth knowing before you commit. Most fixed-rate products cap penalty-free overpayments at 10% of the outstanding balance each year, and exceeding that triggers an early repayment charge that can wipe out the benefit. And an overpayment is generally irreversible unless your lender offers a borrow-back facility, so an emergency fund normally comes first.
Fixed, tracker and standard variable
A fixed rate holds your payment steady for an agreed period, usually two or five years, after which you revert to the lender's standard variable rate unless you remortgage. A tracker follows the Bank of England base rate by a set margin, so payments move up and down with it. The standard variable rate is the lender's own rate, changeable at will, and it is almost always the most expensive place to sit.
Since this calculator applies one rate for the whole term, a fixed deal is the case it models most faithfully — and only until the fixed period ends. A realistic approach is to run the figures twice: once at your current rate, and once at a materially higher one, to see what the payment would look like if you reverted or remortgaged into a worse market. Around 1.8 million UK fixed-rate deals were due to expire during 2026, which makes that second calculation more than hypothetical for a lot of households.
What this calculator does not cover
It models one rate applied consistently for the whole term, so it will not show the step change when a fixed period ends or a tracker moves. It excludes arrangement, booking, valuation, legal and broker fees, and any early repayment charge. It assumes interest is calculated monthly, while some lenders calculate daily, which shifts the total slightly.
It does not carry out an affordability assessment. A lender will stress-test your income against a materially higher rate, examine your credit file and outgoings, and may lend considerably less than these figures imply you could service. Stamp Duty here covers standard residential purchases in England and Northern Ireland only, and leaves aside the non-resident surcharge, shared ownership, linked transactions, multiple dwellings and company purchases, all of which follow separate rules.
Frequently asked questions
How much are the monthly repayments on a £300,000 mortgage?
At 5.24% over 30 years, £300,000 borrowed costs about £1,654 a month on a repayment basis. The same debt over 25 years costs roughly £1,796 a month, and on interest-only it would be £1,310 with none of the capital repaid. Move the rate a single percentage point and the 30-year figure changes by around £190 a month, which is why the rate matters more than almost anything else you can negotiate.
How is a UK mortgage repayment calculated?
Lenders use the standard annuity formula. The annual rate is divided by twelve to get a monthly rate, and the payment is set at the level that clears the balance exactly at the end of the term. Each month, interest is charged on whatever is still owed, and the remainder of your payment reduces the debt. Because the balance falls every month, the interest share shrinks and the capital share grows, which is why progress feels slow at first and then accelerates.
What is the difference between a repayment and an interest-only mortgage?
A repayment mortgage clears both the interest and the capital, so you own the property outright at the end of the term. An interest-only mortgage covers just the interest, so the monthly cost is lower but the original debt is still there on the final day and has to be repaid in full from savings, an investment or a sale. Interest-only lending to residential buyers is tightly restricted now and normally requires evidence of a credible repayment plan.
How much Stamp Duty will I pay?
On a £340,000 purchase in England or Northern Ireland, a home mover pays £7,000, a first-time buyer pays £2,000, and someone buying an additional property pays £24,000 once the 5% surcharge is added. The tax is charged in slices, so a higher band never applies to the whole price. Scotland and Wales use separate taxes with their own thresholds.
Do first-time buyers pay Stamp Duty?
Not on the first £300,000, provided the property costs £500,000 or less. Between £300,001 and £500,000 the rate on that portion is 5%. Above £500,000 the relief is withdrawn completely, not tapered, so a first-time buyer paying £505,000 is taxed exactly like anyone else. Everyone buying with you must also be a first-time buyer for the relief to apply.
How much does overpaying a mortgage save?
More than most people expect, because every extra pound goes straight at the balance and stops accruing interest for the rest of the term. On £289,000 at 5.24% over 30 years, £150 a month extra saves about £59,000 in interest and clears the debt more than five years early. Most fixed deals allow overpayments of up to 10% of the balance each year before early repayment charges apply, so check your own limit first.
Is a 25-year or a 30-year mortgage term better?
A shorter term costs more each month and far less overall. On £289,000 at 5.24%, 25 years costs about £1,730 a month against £1,594 over 30 years, but saves roughly £55,000 in total interest. The longer term is not a mistake if it is what makes the payment affordable, and you can always overpay later, but the extra five years is genuinely expensive.
Why is my lender's quote different from this calculator?
Small differences are normal. Lenders may charge interest daily rather than monthly, round the payment to the nearest pound, add product or arrangement fees to the loan, and treat the first month differently depending on the completion date. Any of those shifts the figure slightly. The mortgage illustration you are given is the document that governs your actual payments.
These figures are estimates for general information and are not financial, mortgage or tax advice. Stamp Duty rates reflect the England and Northern Ireland bands in force from 1 April 2025, published by HMRC. Your lender's mortgage illustration and your conveyancer's Stamp Duty return are the documents that govern your purchase.