UK Mortgage Affordability Calculator — Methodology

Last verified 5 May 2026 against FCA MCOB 11 affordability and stress-test rules. Calculator at /uk/property/mortgage-affordability-calculator/.

What this calculator does

Estimates the mortgage borrowing a UK lender is likely to offer based on income multiples (typically 4.5x) and affordability of monthly payments under a stressed interest rate (product rate + 3%).

The formula

Income-multiple cap:
  cap_income = (grossSalary + partnerSalary) × multiple

Affordability cap:
  monthly_disposable = (netMonthlyIncome - committedOutgoings)
                     × affordabilityFactor
  stress_rate = productRate + 0.03
  r = stress_rate / 12
  n = termYears × 12
  cap_afford = monthly_disposable × (1 - (1+r)^-n) / r

maxLoan = min(cap_income, cap_afford)

Authoritative sources

• FCA MCOB 11: responsible lending
• Bank of England — withdrawal of affordability test (Aug 2022)
• MoneyHelper — how much can you borrow?

Assumptions and limitations

• Uses a 4.5x income multiple by default; some lenders go higher for high earners and lower for sole low-income applicants.
• Stress rate is product rate +3 percentage points, reflecting standard FCA MCOB 11 expectations.
• Does not model specific lender criteria (e.g. Nationwide Helping Hand, joint borrower sole proprietor, contractor day-rate × 5 × 46).
• Committed outgoings include credit cards, loans, childcare, and persistent subscriptions — but lenders interpret 'committed' differently.
• Does not assess deposit, loan-to-value caps, or cladding / EWS1 restrictions.

Source data

This calculator is rate-free — it does not depend on a tax-year rate table. The methodology and assumptions above are the source of truth.

Verification log

• 5 May 2026: Affordability formula and stress assumption reviewed during audit cycle 5