Mortgage Affordability Calculator

How much can you actually borrow? This calculator applies the three constraints UK and Gibraltar lenders typically use — income multiple, monthly affordability, and a stress-rate check — and shows you which one is the binding limit.

The three constraints — what lenders actually check

Mortgage affordability isn't a single number. UK and Gibraltar lenders apply three independent tests and approve the smallest result:

1. Income multiple (loan-to-income cap)

Most UK lenders cap lending at 4 to 4.5× a single applicant's gross annual income, or 4 to 4.5× the combined income for joint applications. A few specialists go to 5× for high earners or specific professions (doctors, lawyers, certain public-sector roles), but those are exceptions. The Bank of England also caps the proportion of mortgages a UK lender can issue above 4.5× — currently no more than 15% of new lending.

2. Monthly affordability

Even if the income multiple allows a bigger loan, the lender will stress-test whether you can actually afford the monthly payment. The standard rule of thumb: your total mortgage payment plus existing debts should not exceed 35-45% of net monthly income. This is the “debt-to-income” (DTI) check — high DTI is the single strongest predictor of default in lender data.

3. Stress rate test

Since 2014, UK regulators have required lenders to test whether you could still afford the payment if interest rates rose by 3 percentage points above the contracted rate during the first 5 years of the mortgage. (The Bank of England relaxed this rule in August 2022, but most lenders kept it as policy.) If you can't afford the stressed payment, the loan size shrinks until you can.

Worked example

Single applicant, £55,000 gross income, £40,000 deposit, 30-year term, 5.5% rate, with £250/month of existing debt minimums. Estimated net monthly income: about £3,300.

• Income multiple (4.5×): £55,000 × 4.5 = £247,500.
• Affordability (40% of net minus debts): capacity = £3,300 × 0.40 − £250 = £1,070/month. Max loan at 5.5% over 30 years ≈ £188,500.
• Stress test (8.5%): same £1,070/month at 8.5% over 30 years ≈ £139,200.

Binding constraint: the stress test, at £139,200. Add the £40,000 deposit and the max property price is about £179,200. Notice: the headline 4.5× income multiple would have allowed a £287,500 property — the stress test cuts that by more than £100k. This is why the “how much can I borrow” calculators you see on bank websites often quote bigger numbers than your underwriter eventually offers.

What affects your real number

• Existing debts. Every £100/month of debt minimums reduces your borrowing capacity by roughly £15,000-20,000. Paying off a credit card before applying often increases your offer more than the card balance itself.
• Credit score. Lower credit scores mean higher rates and tighter affordability — sometimes a hard refusal. Check yours before applying.
• Loan term. 35-year and 40-year mortgages reduce the monthly payment and stretch what you can “afford,” but pay vastly more interest over the lifetime. The same £200,000 mortgage at 5.5% costs £409,400 over 30 years vs £465,200 over 40 — £56,000 extra for the longer term.
• Variable vs fixed. Variable-rate mortgages may pass affordability easier in low-rate environments but expose you to the stress scenario for real if rates rise.
• Joint vs single. Joint applications usually unlock more borrowing, but lenders now scrutinise the lower earner's income; a couple where one earns £80k and the other £15k won't borrow as if they earn £95k each.
• Self-employment. Most lenders require 2-3 years of accounts, and use the average rather than the most recent year. Self-employed applicants typically need a larger deposit and a more conservative LTV.

LTV — why the deposit ratio matters as much as the loan size

Loan-to-value (LTV) is your loan divided by the property price: £160,000 borrowed on a £200,000 home = 80% LTV. Lower LTV gets you better rates because the lender's risk is lower. Common UK bands:

• 60% LTV — best rates, usually requires 40% deposit.
• 75% LTV — competitive rates.
• 85% LTV — typical first-time buyer territory, slightly higher rates.
• 90% LTV — still mainstream but rates step up noticeably.
• 95% LTV — small deposit (5%), specialist products, highest rates.
• 100% LTV — rare, usually requires a guarantor.

A 5% larger deposit can reduce your interest rate by 0.3-0.5% — over a 30-year £200,000 mortgage, that's £15,000-25,000 in saved interest. Sometimes it's worth delaying purchase by 6-12 months just to cross an LTV threshold.

Beyond the lender's calculation: should you?

Passing the lender's affordability test does not mean the mortgage is wise. A 40% DTI is the upper end of what banks will lend — it leaves you with 60% of net income to cover everything else including council tax, utilities, food, transport, savings, and life. At that DTI, a job loss or major repair becomes a crisis quickly. Most personal-finance professionals recommend keeping total housing cost (mortgage + tax + insurance + maintenance) under 28-30% of gross income, which is roughly 35% of net.

What this calculator does NOT include

• Council tax, building insurance, ground rent, service charge
• Property taxes (UK Stamp Duty, Gibraltar Stamp Duty)
• Survey, legal, and broker fees
• Maintenance and repair reserves (rule of thumb: 1% of property value per year)
• Personal living costs beyond declared monthly debt minimums
• Lender-specific scorecard adjustments (different banks weight criteria differently)
• First-time buyer schemes (Help to Buy successors, shared ownership, etc.)

Disclaimer

Educational only. Real lender decisions depend on credit profile, full income verification, property valuation, and the bank's underwriter. Use this calculator to sense-check what's in range; talk to a regulated mortgage broker before house-hunting in earnest.