Every mortgage calculator gives you a monthly payment. That number is the least interesting thing it computes, and fixating on it is how people end up surprised five years in. This guide runs one example all the way through the Mortgage Calculator and pulls out the four figures that actually change decisions.
The example
A £250,000 property, a £25,000 deposit (10%), so a £225,000 loan at 5.5% over 25 years. The calculator returns a monthly principal-and-interest payment of £1,381.70.
Fine. Now the numbers underneath it.
Figure one: the total, which nobody quotes you
£1,381.70 a month for 300 months is £414,509. Against a £225,000 loan, that's £189,509 of interest — you are paying for the house roughly 1.84 times.
This is not a scandal, it's what borrowing money for a quarter of a century costs, and it's why the total is worth looking at once before you stop looking at it. Nobody in the transaction has any particular incentive to say it out loud.
Figure two: where the first payment actually goes
Of that first £1,381.70:
- £1,031.25 is interest — the £225,000 balance times the monthly rate (5.5% ÷ 12 = 0.4583%).
- £350.45 reduces the loan.
Just over a quarter of your first payment is buying the house. The rest is rent on the money. This isn't a trick — it falls straight out of the amortisation formula, because interest is charged on whatever you still owe, and at the start you still owe everything.
Follow it forward in the calculator's amortisation table and the shape becomes obvious:
- After 1 year: you've paid £16,580 and the balance is £220,687. You've cleared £4,313 of the loan.
- After 5 years: £82,902 paid, balance £200,861. You've cleared £24,139 — under 11% of the loan, in a fifth of the term.
- After 10 years: £165,804 paid, balance £169,101. Only now are you around a quarter of the way through the debt.
The crossover, where more of each payment finally goes to principal than to interest, lands at month 150 on these numbers — exactly halfway through the term. Everything before it is front-loaded interest.
Why this matters practically: if there's a realistic chance you'll move within five years, you will have built almost no equity through repayments — your position will be driven almost entirely by what happened to the property's value and by your deposit. That's a genuinely different bet from the one people think they're making, and the amortisation table is where you can see it.
Figure three: what an overpayment actually buys
Here's the one that surprises people in the useful direction. Because every extra pound comes off the balance, and the balance is what generates interest, overpayments compound in your favour for the entire remaining term.
On the same loan:
- +£100 a month: paid off in about 21 years 9 months instead of 25 — a little over three years early, saving roughly £29,000 in interest.
- +£200 a month: paid off in about 19 years 3 months, saving roughly £50,000.
A hundred pounds a month is £26,100 handed over across those 21 years, and it removes about £29,000 of interest. That is a better guaranteed return than most people will find anywhere else, and it's available without talking to anyone.
The way to model this in the calculator is to compare scenarios: run the baseline, note the total interest, then re-run with a shorter term that produces roughly your payment-plus-overpayment as the monthly figure. The two aren't identical — a formal shorter term is a commitment, a voluntary overpayment isn't — but the interest arithmetic lines up closely enough to make the decision with.
Check first: many fixed-rate deals cap overpayments (10% of the balance a year is a common limit) and charge early repayment fees beyond it. The calculator doesn't know your lender's terms. Your mortgage offer does.
Figure four: 25 years versus 30
Same loan over 30 years: £1,277.53 a month. That's £104 a month cheaper, which is the number an affordability conversation will focus on.
The total is £459,909 — £234,909 of interest, against £189,509 over 25 years. The longer term costs about £45,400 more to save £104 a month.
Which is not automatically the wrong choice. A lower committed payment is real breathing room, and a 30-year term you can comfortably overpay may beat a 25-year term that leaves you with nothing spare. But it should be a decision you make with the £45,400 in view, not one you drift into because the monthly figure looked friendlier.
This is also where the Age Calculator earns a place in a home-buying spreadsheet, oddly enough. Enter your date of birth and add the term: a 30-year mortgage taken at 41 finishes at 71. Whether that's fine or alarming depends entirely on your pension plans, but a surprising number of people only run that subtraction after signing.
The unit conversions you will hit within ten minutes
Property listings are a mess of mixed units, and the Unit Converter exists partly because of them. The conversions that come up constantly:
- Square feet to square metres. One square metre is 10.7639 square feet. So a "1,200 sq ft" flat is about 111.5 m², and a 90 m² flat is about 969 sq ft. Halving or doubling in your head gets this badly wrong, because the factor is nearly eleven, not three.
- Acres and hectares. An acre is 4,046.86 m², or 43,560 sq ft; a hectare is 10,000 m², about 2.47 acres. Useful the moment a listing mentions land.
- Feet and inches to metres for ceiling heights, doorways and whether the sofa fits — an inch is exactly 25.4 mm, and the converter uses that exact definition rather than an approximation.
The reason to use a converter rather than a rough mental factor is that price-per-square-metre comparisons are one of the few genuinely objective ways to compare two properties, and they fall apart entirely if one of the areas is 8% out.
What the calculator can't do
It models a fixed rate held to term. It knows nothing about tracker or variable rates, offset accounts, early repayment charges, private mortgage insurance, service charges, ground rent, stamp duty, survey and legal fees, or what a lender will actually offer you once they've seen your paperwork. Property tax and insurance can be entered so the monthly figure reflects a real outgoing rather than principal and interest alone, but they're your estimates, not quotes.
Use it to compare scenarios against each other, which is what it's good at. Use a lender for what you'll actually pay.
The amortisation formula is written out in full on the methodology page if you want to reproduce any of the figures above yourself.