About the UK Fixed vs Variable Mortgage Calculator
Most UK mortgages fix or track for two, three or five years and then revert to the lender’s Standard Variable Rate (SVR) or to whatever deal you remortgage onto. This calculator compares two paths side by side: keeping one fixed rate for the whole term, versus taking a cheaper initial deal and then paying a variable rate for the rest of the term. It shows the monthly payment on each, the total interest you would pay, and the gap between them.
It is designed for UK buyers and remortgagers weighing certainty against flexibility — for example a 10-year or lifetime fix against a 2- or 5-year fix or tracker. The break-even variable rate tells you how high rates would need to be after the initial deal for the long fix to win, which is a clearer way to judge the risk than guessing where the Bank of England base rate will go.
Both options are repayment (capital and interest) mortgages over the same term. Arrangement fees, early repayment charges and product switching are not included, and the variable rate is assumed to stay flat after the initial deal ends.
With the default inputs, the extra cost of the variable path is £37,721.24. Change any value above to recalculate instantly.
How to use the uk fixed vs variable mortgage calculator
- 1Enter the property price, your deposit and the mortgage term.
- 2Enter the long-term fixed rate you have been offered.
- 3Enter the initial deal rate and how many years it lasts.
- 4Enter the variable rate you expect afterwards (SVR or future remortgage rate).
- 5Compare total interest and check the break-even variable rate.
Formula and method
The fixed option uses the standard repayment formula once: loan P, monthly rate r (annual rate ÷ 12) and n monthly payments over the whole term. The deal-then-variable option starts with the same formula at the initial deal rate over the full term; when the deal ends, the outstanding balance is re-amortised at the variable rate over the months left, which is how UK lenders recalculate payments on reversion.
Total interest is the sum of monthly interest charges on each path. The break-even variable rate is found numerically: it is the reversion rate at which both paths cost exactly the same total interest. Fees, early repayment charges and future rate changes are not modelled.
- M
- Monthly repayment
- P
- Loan balance being repaid (property price − deposit at the start)
- r
- Monthly interest rate (annual rate ÷ 12 ÷ 100)
- n
- Number of monthly payments remaining
Worked examples
£240k loan: 4.9% fixed vs 5 years at 4.3% then 6.5% SVR
Fixing £240,000 at 4.9% for 25 years costs £1,389.07 a month and about £176,721 interest. The 4.3% five-year deal starts £82 a month cheaper, but reverting to a 6.5% SVR raises the payment to about £1,567 and total interest to about £214,442 — roughly £37,700 more. The variable path only wins if rates after year five average below about 5.2%.
2-year tracker at 4.0% then remortgage at 4.5%
If you expect to remortgage at around 4.5% after a 4.0% two-year tracker, the variable path costs about £157,356 in interest, some £19,400 less than the 4.9% long fix. The payment rises only modestly, from £1,267 to about £1,330, when the tracker ends, and the long fix would only win if rates after year two averaged above about 5.04%.
Frequently asked questions
Is it better to fix or go variable on a UK mortgage?+
Fixing gives certain payments and protects you if rates rise, usually at a slightly higher starting rate. A tracker or variable deal can be cheaper if rates fall but exposes your budget to rises. Use the break-even rate to see how much rates would need to move.
What happens when my fixed rate ends?+
Your mortgage moves to the lender’s Standard Variable Rate, which is usually well above new-customer deals. Most borrowers switch to a new product or remortgage before the deal ends to avoid paying the SVR.
What is the difference between a tracker and an SVR?+
A tracker follows the Bank of England base rate plus a fixed margin, so it moves only when the base rate changes. An SVR is set by the lender and can change at any time, although it usually moves with the base rate.
Are there early repayment charges on fixed-rate mortgages?+
Most fixed deals charge an early repayment charge (often 1%–5% of the balance) if you leave or overpay beyond the allowance during the fixed period. Long fixes can be expensive to exit, so consider how likely you are to move.
How long should I fix my mortgage for?+
Two-year fixes suit people who expect rates to fall or plan to move soon; five-year and longer fixes suit those who value stable payments. Compare the total cost over the period you expect to keep the mortgage, including fees.
Results are estimates for educational purposes and are not financial advice. Rates, fees and terms vary — confirm with your lender or a licensed advisor before making decisions.