Guide · United Kingdom
How Is Mortgage Affordability Calculated in the UK? (2026)
Last updated 2026-04-25 · Published 2026-04-25
A practical companion guide to the UK affordability formula: income times multiple minus commitments, stress-tested payment checks, and why lenders return different borrowing limits.
The core formula UK borrowers should know
If you are asking how is mortgage affordability calculated, most UK lenders begin with a simple headline equation: affordability estimate = (gross annual income x income multiple) - commitments. This first pass is fast, easy to explain, and useful for setting a rough budget range before you speak to a broker.
In practice, lenders define commitments as your regular monthly outgoings that already exist before the mortgage starts: car finance, personal loans, credit card assumptions, childcare, maintenance, and similar fixed costs. Some lenders subtract commitments directly in annual form, while others convert commitments into a payment-cap model and then back into a maximum loan. The method changes, but the logic is the same: higher ongoing commitments reduce what you can safely borrow.
Worked example: £45,000 salary, £300 monthly commitments, 4.5x multiple
Step 1 is the income-multiple headline. With a gross salary of £45,000 and a 4.5x multiple, the starting figure is £45,000 x 4.5 = £202,500. If you stopped here, you might assume your maximum mortgage is about £202,500.
Step 2 applies commitments. Monthly commitments of £300 equal £3,600 per year. Using the companion formula in this guide, commitment-adjusted borrowing is £202,500 - £3,600 = £198,900. That does not guarantee approval, but it is a better estimate than income multiple alone because it reflects existing debt pressure.
Step 3 is policy context. A lender may still adjust this result for credit profile, employment type, term, dependants, and property factors. So £198,900 is a planning number, not an offer letter. It is useful because it makes your assumptions explicit before you run lender-specific checks.
How the stress test rate changes the calculation
The formula above is only the first gate. UK lenders then test whether the payment remains affordable at a stressed rate, not only at the product rate. Using the worked example value (£198,900 over 25 years), repayment is about £1,139.69 per month at 4.8%, but about £1,469.85 per month at a 7.5% stress rate. That jump of roughly £330 per month is exactly why a case that looks fine at headline rates can fail affordability policy.
This is also why two applicants with similar salaries can receive different answers when rates move or lender stress assumptions differ. The stress test can become the binding constraint, overriding the income-multiple figure. If your budget is tight, even a small increase in stress rate can cut borrowing power materially.
Why results differ between lenders
Lenders do not all use identical affordability engines. One bank might accept a 4.5x multiple for your profile, another may cap at 4.0x. One may treat bonus income conservatively, another may include a larger share with evidence. One may use a tougher stressed-rate floor. Each of those decisions shifts your final number even if your salary is unchanged.
Commitment treatment also varies. Some lenders use assumed credit-card costs from your limit, not your balance. Others apply different living-cost benchmarks by household size and region. So when borrowers ask how is mortgage affordability calculated, the honest answer is two-layered: there is a common formula shape, then a lender-specific policy overlay that changes the output.
Common mistakes when applying the formula yourself
The biggest error is mixing gross and net income. Income multiples are usually based on gross annual income, while your real-life budget feels like net monthly pay. If you combine gross income with net-style expense assumptions, the result can look stronger than your true comfort zone. Keep the math framework consistent from start to finish.
A second mistake is under-reporting commitments or ignoring near-term changes. If a 0% credit card deal expires, childcare starts, or a car loan begins before completion, your affordability can move quickly. Run at least two scenarios in your planning: a base case and a stressed case with higher commitments or rates. That makes lender feedback more predictable and reduces surprises after decision in principle.
Try the numbers yourself
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Go to calculator →Frequently asked questions
How is mortgage affordability calculated in the UK for first-time buyers?
The usual starting point is gross income times a lender multiple, then reduced for existing commitments. After that, lenders run stressed-payment checks and policy rules. So the first-time buyer answer is not just one number; it is a formula plus underwriting checks.
How is mortgage affordability calculated when I have monthly commitments?
Commitments are usually treated as costs that reduce borrowing power. In a simplified companion formula, affordability estimate = (income x multiple) - commitments. For example, £300 monthly commitments can materially lower the headline loan once annualized and stress-tested.
How is mortgage affordability calculated under stress test rates?
Lenders calculate whether you could still pay if rates rise. They model repayments at a stressed rate that is often above the product rate, and if the stressed payment is too high relative to policy limits, maximum borrowing is reduced even when income multiple looks acceptable.
How is mortgage affordability calculated differently by each lender?
Lenders differ on income multiples, bonus treatment, stress-rate assumptions, and how commitments are modeled. Because those policy inputs vary, affordability outputs vary too. That is why broker-led comparison across lenders is often valuable.
Educational content only—not mortgage, tax, or legal advice. Confirm any decision with a licensed professional in your jurisdiction.