Take a £350,000 home, 25% down (£87,500), and a 5-year fixed rate of 4.48% — a rate MortgagePro Global draws from Bank of England context — on a 25-year repayment mortgage. The £262,500 loan works out to roughly £1,456 a month. That number is solid for exactly five years. After that, it resets to whatever rate is available at the time, because UK mortgages don't lock a rate for the full term the way US mortgages typically do.
Fixed Terms, Not Fixed Loans
A UK "5-year fixed" mortgage means the rate is fixed for 5 years, against a repayment schedule that's usually spread over 25-35 years. When the fixed period ends, the mortgage doesn't finish — it rolls onto the lender's standard variable rate (SVR), typically much higher, unless you actively remortgage or take a product transfer beforehand. Two-year fixes are just as common as five-year ones in the UK market, meaning some borrowers reprice even more often.
The Math
Standard UK repayment mortgages use monthly compounding, the same amortization formula used elsewhere:
Where P is the loan amount, r is the monthly rate (annual ÷ 12), and n is the number of monthly payments.
Worked Example
| Input | Value |
|---|---|
| Property price | £350,000 |
| Deposit (25%) | £87,500 |
| Mortgage amount | £262,500 |
| Rate | 4.48% (5-yr fixed) |
| Term | 25 years (300 payments) |
| Monthly Payment | ≈ £1,456 |
Common Mistakes
Borrowers frequently treat their fixed rate as permanent and don't plan for what happens when it ends — letting the mortgage lapse onto the lender's SVR is usually the most expensive outcome available, often several percentage points above any fixed deal on the market.
Borrowers also compare only headline rates between a 2-year and 5-year fix without weighing the tradeoff: a 2-year fix often has a slightly lower rate but exposes you to repricing risk twice as often over a decade; a 5-year fix trades a bit of rate for more payment certainty.
A third mistake: forgetting product fees. Many UK mortgage deals carry an arrangement fee (often £999-£2,000, sometimes a percentage of the loan) that changes the true cost comparison between two seemingly similar rates.
Where This Calculator Has Limits
It assumes a standard repayment mortgage — interest-only mortgages calculate very differently (see our Amortization tool for that distinction). It also can't predict what rate will be available when this fix ends, which is the single biggest variable in the true lifetime cost of a UK mortgage.
Frequently Asked Questions
Does my rate ever stay fixed for the whole mortgage?
Not on a standard UK residential mortgage — fixed periods of 2, 3, 5, or occasionally 10 years are common, but the underlying mortgage term (25-35 years) is much longer than any single fix.
What happens if I do nothing when my fix ends?
You automatically roll onto the lender's standard variable rate, which is usually higher than available fixed or tracker deals — most borrowers remortgage or take a product transfer before this happens.
Is a 5-year fix always safer than a 2-year fix?
It offers more payment certainty, but if rates fall during your fixed period, you're locked out of the lower rate until your term ends — there's a genuine tradeoff either way.
Does a bigger deposit always get me a better rate?
Generally yes — crossing certain loan-to-value thresholds (75%, 60%, etc.) typically unlocks better pricing tiers, covered in our LTV Calculator.
Can I overpay a fixed-rate mortgage?
Usually yes, up to an annual allowance (commonly 10% of the balance) without penalty — exceeding it can trigger an early repayment charge.
Related Tools
Stamp Duty (SDLT) Calculator · LTV Calculator · Remortgage Tool · Affordability Calculator
Educational content, not financial advice. Rates and figures are illustrative and change regularly — confirm current numbers with a licensed UK mortgage adviser or lender. Written by the MortgagePro Global team.