Hundred SummersMortgage affordability calculator: how much could you borrow?
The two tests lenders actually run, the income multiple and the affordability check at a stressed rate, plus your deposit and the 95% loan-to-value cap, with the one that binds named and the home price it points to. Your figures never leave your browser.
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Worked example
Computed when this page was built by the same engine that runs the calculator above, so the figures below and the live tool can never disagree. Change any input above to see your own.
| Inputs | |
|---|---|
| Incomes | £45,000 + £30,000 |
| Deposit | £40,000 |
| Monthly commitments | £300 |
| Rate / term | 4.5% / 30 years |
| Income multiple | 4.5× |
| Results | |
|---|---|
| Income multiple allows | £337,500 |
| Affordability at 7.5% stressed allows | £239,602 |
| Deposit at 95% LTV allows | £760,000 |
| Indicative maximum loan | £239,602 (limited by the affordability at the stressed rate) |
| Home price it points to | £279,602 at 86% LTV |
| Monthly payment at the real rate | £1,214 |
The next question
What will the stamp duty be? →Stamp duty calculator
Or answer it with everything else: the full Hundred Summers plan connects your pay, pensions, savings, property, tax and spending year by year, and stress-tests them across 500 simulated market histories.
Read the guides
- Stamp duty when you move: three taxes and the buy-before-you-sell surcharge →SDLT, LBTT and LTT side by side at 2026/27 rates, worked figures for a £300,000 home in each nation, why the additional-property surcharge catches ordinary movers, and the refund window if the old home sells later.
- Your mortgage: how the payment is built, what the term costs, and what a rate change does →Why half of every early payment is interest, what stretching the term to 35 years really costs, the interest-only trap, and how a two-point rate rise moves the monthly figure, with the arithmetic behind each.
The two tests, and why the smaller one wins
The loan-to-income cap is the headline: 4.5× joint gross income at most lenders, with the regulator limiting how much of a lender’s book can exceed that. The affordability test is the one that catches people: the repayment is calculated at a stressed rate (the product rate plus around three points, with a floor), and must fit inside take-home pay after loans, card minimums, childcare and maintenance, leaving room for living costs. On modest incomes with commitments, the affordability test binds well before the multiple does; on high incomes with no debt, the multiple binds. Then the deposit sets the price: with 95% the maximum loan-to-value, a £20,000 deposit caps the loan at £380,000 whatever your income.
What lenders count
Salary counts in full; regular bonus, overtime and commission usually at 50–100% depending on the lender; self-employed income as an average of the last two or three years’ accounts. Take-home here uses the Hundred Summers engine’s current tax and National Insurance. What the loan then costs, and how overpaying or remortgaging changes it, is the job of the repayment, overpayment and remortgage calculators; the stamp duty calculator prices the purchase tax, and in the Hundred Summers app a purchase is a financed-purchase event that lands in every projected year.
Sources
- Loan-to-income limits: Bank of England Financial Policy Committee, financial stability measures (the 15% flow limit on loans above 4.5× income).
- Affordability and stress testing: FCA Mortgage Conduct of Business rules (MCOB 11.6), summarised for consumers at MoneyHelper.
- Income tax and NI: HMRC Income Tax rates and National Insurance rates.