How mortgage affordability actually works
Every UK mortgage lender asks two questions before offering you a loan: how much would you like to borrow, and can you comfortably afford the repayments? The first question is usually answered with an income multiple — a simple ratio of loan size to annual income, commonly somewhere between 4 and 5 times your income, occasionally higher for certain professions or larger deposits. The second, more important question is answered with a full affordability assessment that looks at your actual spending, existing debts, dependants, and how your finances would hold up if interest rates rose.
The calculator above uses the income-multiple method as a starting estimate, because it's the fastest way to get a ballpark figure without a full application. It combines your income (and a second applicant's income, if you're buying jointly), applies your chosen multiple, then reduces the result to account for existing monthly commitments such as car finance, credit card minimum payments, student loan deductions or childcare costs. Add your deposit to the estimated maximum loan and you get an estimated maximum property price, along with the loan-to-value (LTV) ratio that implies.
Why lenders use income multiples in the first place
Income multiples exist because they're a quick, standardised proxy for affordability that's easy to compare across applicants. A single person earning £40,000 with no debts is a very different lending proposition from a couple earning £40,000 combined with two car loans and childcare costs, but both might initially be quoted a similar multiple before the deeper underwriting process kicks in. Since the 2014 Mortgage Market Review, UK regulation has required lenders to go well beyond a flat multiple: they must also run an affordability stress test, checking whether you could still cope if your mortgage rate rose by a specified margin above the rate you'd actually be paying. This is why two people with identical incomes and an identical income multiple can be offered very different loan sizes once their actual spending is taken into account.
What pulls your borrowing power down
The single biggest swing factor in most affordability assessments, after income, is existing debt. Every pound of committed monthly outgoing — a car finance agreement, a personal loan, credit card balances you don't clear in full, a student loan deduction, ongoing childcare payments, even spousal maintenance — reduces the amount a lender considers you can safely put towards a mortgage. Lenders don't apply a single universal formula for this; some use detailed budget modelling based on your bank statements, others apply broader affordability calculators. Our estimate above applies a simplified deduction, multiplying your monthly outgoings by an approximate annual multiplier, to give a directionally sensible adjustment rather than a lender-accurate one.
Credit history matters too, even though it doesn't feature in this calculator. A strong credit score with a clean repayment history can unlock higher income multiples and better rates from some lenders; missed payments, high credit utilisation or a County Court Judgment can restrict your options considerably, sometimes regardless of how comfortable your income multiple looks on paper. If your credit score needs work before you apply, it's usually worth addressing that first, since the improvement can take months to show up on your file.
Deposit size and loan-to-value
Your deposit does two jobs in a mortgage application: it reduces the amount you need to borrow, and it sets your loan-to-value ratio, which is one of the main factors driving the interest rate you're offered. A larger deposit relative to the property price (a lower LTV) generally unlocks cheaper rates, because it represents a smaller risk to the lender if property prices fall. Moving from a 90% LTV to an 85% or 80% LTV can sometimes shift you into a noticeably better pricing tier, which is worth knowing before you commit to a particular price bracket. You can check exactly where any given deposit and price combination falls using our separate loan-to-value calculator.
Turning an estimate into a real mortgage offer
An online affordability estimate like this one is a useful first filter before you start house-hunting or speaking to a broker — it stops you falling in love with a property that's realistically out of reach, and it gives you a sensible budget to work with. But it isn't a mortgage offer, an Agreement in Principle, or a guarantee of any kind. To get an accurate figure, a lender or broker will need to see payslips or accounts, bank statements, proof of any deposit source, and details of your existing commitments, and will run their own underwriting model rather than a simplified multiple.
It's also worth applying for a formal Agreement in Principle before you start offering on properties, since many estate agents expect to see one, and it gives you (and them) more confidence that a mortgage of the size you need is realistically available. Getting one doesn't commit you to that lender, and having a few different lenders' figures side by side is a normal, sensible part of house-hunting rather than something to be worried about. If you're buying for the first time, our first-time buyer guide walks through the whole process end to end, from deposit-saving through to completion.