UK Mortgage Calculator
Compute the monthly repayment, total interest paid, and breakeven affordability on any UK mortgage. The calculator runs entirely in your browser — nothing you enter is sent to a server or third-party analytics endpoint.
Repayment & affordability calculator
Calculator runs entirely in your browser. Nothing is sent to a server. Numbers update as you type.
How the calculator works
The monthly repayment uses the standard UK amortisation formula: M = P × (r × (1 + r)n) / ((1 + r)n − 1), where P is the loan amount, r is the monthly interest rate, and n is the term in months. For interest-only mortgages, the monthly payment is simply P × r with the principal due at the end of the term.
The affordability check applies UK lender stress-test rules: the borrower must be able to afford the payment at base rate + 1 percentage point (currently 5.50%) with total monthly debt service fitting within roughly 35% of net household income.
Worked example scenarios
Five representative UK mortgage scenarios, each computed from the standard amortisation formula at the prevailing base rate plus a typical product margin.
First-time buyer on £50k in London
- Income:
- £50,000/yr
- Deposit:
- £30,000
- Target price:
- £285,000
- Term:
- 30 years
- Rate:
- 5.20%
- Monthly:
- £1,400
- Band:
- stretched
Remortgaging £200k at the end of a 2-year fix
- Income:
- £70,000/yr
- Deposit:
- £100,000
- Target price:
- £300,000
- Term:
- 22 years
- Rate:
- 4.85%
- Monthly:
- £1,234
- Band:
- comfortable
Buy-to-let investor in Leeds
- Income:
- £75,000/yr
- Deposit:
- £65,000
- Target price:
- £175,000
- Term:
- 25 years
- Rate:
- 5.40%
- Monthly:
- £669
- Band:
- comfortable
Couple earning £90k in Manchester
- Income:
- £90,000/yr
- Deposit:
- £45,000
- Target price:
- £320,000
- Term:
- 25 years
- Rate:
- 4.90%
- Monthly:
- £1,592
- Band:
- comfortable
High earner on £150k in Cambridge
- Income:
- £150,000/yr
- Deposit:
- £100,000
- Target price:
- £525,000
- Term:
- 25 years
- Rate:
- 4.65%
- Monthly:
- £2,399
- Band:
- comfortable
Common calculator questions
Why does my actual repayment differ from the calculator?
The calculator uses the contractual amortisation formula on a clean loan. Real mortgages add product fees (typically £499–£1,499), valuation fees, broker fees, and (for high-LTV deals) higher capital cost passed through into the headline rate. Lenders also vary in how they handle interest accrual day-counts. Differences of £5–£20 per month against a real mortgage statement are normal; differences above £50 typically reflect a different product type or term assumption.
How is the affordability ceiling computed?
The household borrowing ceiling uses the lower of the loan-to-income ceiling (typically 4.5× gross household income, occasionally extended to 5× for higher earners) and the stress-test ceiling (the loan size for which the monthly payment at base rate + 1 percentage point fits within roughly 35% of net household income after tax and existing debt service). UK lenders publish slight variations of this; the calculator reflects the mainstream consensus.
Why does the calculator stress at base + 1, not base + 3?
Until 2022 the FCA required all UK lenders to stress at the higher of the contract rate or the lender’s reversion rate plus 3 percentage points (MCOB 11.6.18R). In 2022 the FCA removed the specific 3% stress requirement, leaving lenders to set an appropriate stress at their own discretion. Most lenders settled on base rate + 1 percentage point at the time of the offer, which is what the calculator uses. A more conservative borrower or a lender with stricter standards may prefer base + 2.
Does the calculator work for buy-to-let?
Partially. The repayment calculation is identical. The affordability calculation for BTL is different — the test is rental coverage (rent must cover the stressed mortgage payment by 125% for basic-rate taxpayers and 145% for higher-rate). The calculator does not currently include the BTL coverage test; for BTL specifically, see the guide on buy-to-let mortgage rules.
Where can I see the underlying maths?
The repayment formula, the stress-test inputs, and the LTI ceiling are documented in full on the methodology page. Source citations for the base rate, the income multiples (FCA flow caps), and the stress-test rule (MCOB 11.6.18R) are linked from there.
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