Best Mortgage Calculators Compared: Which One Gives You the Real Number?
Get a different figure from every mortgage calculator you try? They're not wrong — they're answering different questions. Here's what each one actually tells you, and which number to trust when.
Plug the same salary, the same property price, and the same deposit into three different mortgage calculators, and you can walk away with three different answers — sometimes tens of thousands of pounds apart on what you can borrow, or over a hundred pounds a month apart on what it'll cost. None of them is wrong. They're built to answer different questions, and knowing which one you're looking at is the difference between a realistic house hunt and an unpleasant surprise at a lender's affordability check.
Key Takeaways
A simple "how much can I borrow" calculator and a full affordability calculator can differ by 10% or more — on a £42,000 salary, a 4.5x income multiple suggests roughly £189,000, while an affordability check that factors in outgoings and a rate stress test often lands closer to £165,000.
That gap changes the monthly number too: at 5.52%, a £189,000 mortgage costs around £1,076 a month, versus £939 on £165,000 — a £137 monthly swing from the same salary alone.
Repayment calculators do the maths correctly for whatever loan, rate, and term you enter — but most don't add stamp duty, buildings insurance, or maintenance, so the "monthly cost" they show is usually £150–£250 lower than the real all-in figure.
Stamp duty lives in its own calculator entirely. First-time buyers pay nothing up to £300,000 and 5% on the portion up to £500,000, but a repayment calculator won't tell you either way — you have to check separately.
The only number that's actually binding is an Agreement in Principle from a real lender. Every calculator before that point — including ours — is a planning estimate, some more conservative than others.
At a Glance — What Each Calculator Actually Tells You
Calculator Type
What It Answers
What It Typically Leaves Out
Income-multiple / "How much can I borrow"
Rough borrowing ceiling (salary × multiple)
Your real outgoings, debts, and any rate stress test
7 min read·September 21, 2026·0
James Calloway
Writer
Full affordability calculator
Realistic borrowing limit after income, debts, spending, and a rate-rise check
Property-specific costs like insurance and maintenance
Repayment calculator
Monthly payment for a loan amount, rate, and term you enter
Stamp duty, insurance, maintenance, product fees
Stamp Duty calculator
Land tax due on a given purchase price
Everything else about affordability
Remortgage calculator
New payment vs your current deal, and whether switching pays off
Early repayment charges unless entered manually
Figures are illustrative examples — always confirm your own numbers with a lender or broker before relying on them.
Why a Borrowing Calculator and an Affordability Calculator Don't Agree
Most "how much can I borrow" calculators use a simple income multiple — commonly up to 4.5x salary. On £42,000, that's roughly £189,000. It's a fast, useful ballpark, but it's not what a lender will actually offer.
Under FCA rules, lenders have to assess whether you could realistically keep affording the mortgage — factoring in your existing debts, regular spending, dependants, and typically some allowance for rates being higher than they are today — not just multiply your salary. Run the same £42,000 income through a full affordability calculator, like the ones on MoneyHelper or a lender's own site, and it's common to see the figure fall closer to £165,000 once real outgoings and a rate buffer are factored in — about 13% lower than the simple multiple suggested.
That's not a rounding error once it hits the monthly payment. At 5.52%, £189,000 over 30 years costs around £1,076 a month; £165,000 costs around £939. Same salary, same rate, £137 a month apart — purely because one calculator stress-tests you and the other doesn't.
The Repayment Calculator Gets the Maths Right — For What You Type In
A repayment calculator, including our Mortgage Calculator, isn't guessing — given a loan amount, rate, and term, the monthly payment it returns is arithmetically correct. The problem is what people leave out when they use it. Buildings insurance and a realistic maintenance budget (commonly estimated at 1–1.5% of the property's value a year) rarely make it into the boxes, which means the number on screen usually understates the real monthly cost by £150–£250, depending on property value.
💡 Get the full picture, not just the mortgage part: run your loan amount and rate through our Mortgage Calculator, then add a realistic insurance and maintenance line on top before comparing it to rent or another property — the repayment figure alone is only part of the true monthly cost.
Stamp Duty Lives in a Separate Calculator Entirely
This is the one people forget most often. A repayment calculator tells you nothing about the one-off cost of buying — because stamp duty isn't part of the mortgage at all, it's a land tax calculated separately on the purchase price. First-time buyers currently pay nothing on the first £300,000 and 5% on the portion between £300,001 and £500,000, with no relief above that. You can check the exact figure for any price on gov.uk's Stamp Duty Land Tax guidance — but a mortgage repayment calculator, however accurate on the monthly figure, simply won't surface it. If your target property sits under £300,000 as a first-time buyer, that particular cost is zero — but you should confirm it, not assume it.
Remortgage Calculators Answer a Different Question Again
If you're not buying but switching, the calculator you need is different again — it's comparing your current deal against a new one, not working out what you can afford from scratch. Do nothing when a fixed deal ends and most lenders move you onto a Standard Variable Rate, currently averaging around 7.13% — roughly £230 a month more on a £250,000 mortgage than a competitive remortgage deal, as we cover in Remortgaging in 2026. A remortgage calculator's job is to catch that gap before it happens, not to tell you how much house you can afford.
So Which Number Should You Actually Trust?
Treat them as a sequence, not a single source of truth. Use an income-multiple calculator for a first ballpark, a full affordability calculator (yours, or your lender's) for a realistic ceiling, a repayment calculator plus insurance and maintenance for the true monthly cost, and gov.uk's own tool for stamp duty. None of them, on their own, is "the real number" — and per the FCA's own guidance to consumers, the only figure that's actually binding is an Agreement in Principle from a real lender, which checks your credit file and verifies your income directly. Everything before that is a planning estimate.
What This Means for You
Getting different numbers from different mortgage calculators isn't a sign that one of them is broken — it's a sign they're built to answer different questions. The income-multiple figure tells you the ceiling; the affordability check tells you the realistic number; the repayment calculator tells you the monthly cost of a specific loan; and none of them include stamp duty unless you check separately. Used together, in the right order, they get you close enough to shop with confidence. Used in isolation, any one of them can leave you house-hunting at a price you were never actually going to be offered.
What You Should Do Next
Run your income through both a simple multiple calculator and a full affordability calculator, and budget around the lower of the two
Add buildings insurance and a realistic maintenance estimate on top of any repayment calculator's monthly figure
Check stamp duty separately on gov.uk for your specific purchase price — don't assume a mortgage calculator has included it
Get an Agreement in Principle before treating any calculator's output as the number you can actually rely on
Mortgage calculators disagree with each other because they're solving different problems — income-multiple tools give a ceiling, affordability calculators stress-test it down, repayment calculators do the monthly maths on whatever you enter, and stamp duty sits in a calculator of its own. None of that makes any of them useless; it just means the "real number" only shows up once you've run all the right ones and gotten an Agreement in Principle to confirm it.
FAQ
Why did I get a much bigger borrowing figure from one calculator than another?
Simple income-multiple calculators (often up to 4.5x salary) don't stress-test affordability the way a full lender assessment does. Factoring in real outgoings and a rate buffer typically brings the figure down — in our example, from around £189,000 to roughly £165,000.
Does a mortgage repayment calculator include stamp duty?
No. Stamp duty is a separate one-off tax calculated on the purchase price, not part of the mortgage itself. Check it independently on gov.uk rather than assuming a repayment calculator has accounted for it.
Why does my repayment calculator figure feel lower than what I'm actually paying?
It's likely only showing the mortgage payment itself. Buildings insurance and a realistic maintenance budget commonly add £150–£250 a month on top, depending on the property's value.
Which calculator gives the number I can actually rely on?
None of them fully, on their own. The only binding figure is an Agreement in Principle from a real lender, which checks your credit file and verifies your income directly.
This article is for informational purposes only and does not constitute financial advice. Mortgage rates, lending criteria, and stamp duty thresholds change frequently — always confirm current figures with a lender, broker, or official government source before making a borrowing decision.
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James Calloway is a personal finance writer and real estate strategist covering housing markets across the UK, US, and beyond. With a decade of experience translating complex financial decisions into plain language, he believes the best money advice sounds like a candid conversation — not a lecture.
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