Most UK mortgages are short fixed-rate deals — 2, 3 or 5 years — which means the rate you pay today is unlikely to be the rate you pay forever. This calculator shows what your monthly payment becomes if interest rates rise by 1%, 2%, 3%, or all the way to 8% (close to the 2023 peak), so you can decide whether your budget would survive.

Why stress testing matters

Since 2014, UK lenders have been required to test affordability against a higher rate than the headline pay rate — usually pay rate + 1% or the lender's reversion rate plus 1%. That's the lender's safety check. This calculator is your own safety check: if your fix ends in two years and rates haven't moved, your payment stays the same. If they've risen by 2%, the payment shown in the "+2%" row is what you'd be paying.

How the calculation works

Each scenario uses the standard repayment-mortgage formula on a capital and interest basis. The monthly payment M for a loan P at monthly rate r over n months is:

M = P × r × (1 + r)ⁿ / ((1 + r)ⁿ − 1)

At a 0% rate the formula degrades — we treat it as simple straight-line repayment (loan ÷ months). This matches what UK lenders show.

What "8%" represents

In late 2023 the average UK two-year fixed mortgage rate peaked near 6.85%, while standard variable rates at several major lenders went above 8%. The Bank of England base rate hit 5.25%. We use 8% as a stress benchmark because it captures the worst-case experience from that cycle plus a small buffer. If your budget can handle 8%, you have substantial cushion against any plausible 2026–2028 rate move.

What to do if the numbers look tight

• Overpay while you can — most fixed deals allow 10% overpayment per year without penalty. Every £1 of overpayment shrinks the balance the next rate is calculated against.
• Lock in early — you can usually secure a new mortgage rate 6 months before your current fix ends. If rates rise in those 6 months, you've already locked the lower one. If they fall, most lenders let you switch to the lower rate.
• Extend the term — going from 25 to 30 years reduces the monthly payment meaningfully (at the cost of total interest paid). Most lenders will allow this at remortgage.
• Speak to a broker — they see deals across the whole market and can flag products with features that suit a tighter budget (10-year fixes, offset, flexible overpayment).

Related calculators

• Mortgage repayment calculator — basic monthly payment for one rate
• Affordability calculator — what you can borrow
• Moving costs calculator — full cost to complete
• Deposit savings calculator — how long until you can buy

Calculation rules last reviewed: 18 May 2026.