Fixed vs Variable Mortgage Rate Calculator
Cost comparison fixed vs variable mortgage.
Compare cost of fixed versus variable mortgage rates across the fixed period, with the break-even point at which variable would win.
What this tool does
This calculator models the total interest cost of a fixed-rate mortgage against a variable-rate mortgage over a specified period. It takes your mortgage balance, the fixed rate offered, the current variable rate, and the length of the fixed period, then estimates what you would pay in interest under each scenario. The output shows the total interest cost for both paths, allowing you to see which option costs less based on today's rates. The fixed rate is the primary driver—higher fixed rates increase fixed-path costs—while changes to the variable rate assumption shift the variable-path cost. A typical use case is comparing a 5-year fixed offer against staying on a variable rate. Note that this is a simplified calculation that assumes rates remain constant and doesn't account for fees, adjustments, or actual payment schedules. Results are for educational illustration only and reflect current conditions, not future rate movements.
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Formula Used
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Disclaimer
Results are estimates for educational purposes only. They do not constitute financial advice. Consult a qualified professional before making financial decisions.
Fixed vs variable rate mortgage choice depends on rate spread and risk tolerance. Fixed gives certainty at typically slight premium. Variable cheaper now but risks rate rises. Calculator shows immediate cost comparison — fixed only wins if variable rises significantly.
A worked example
Try the defaults: mortgage balance of 200,000, fixed rate of 5.5%, current variable rate of 5%, fixed period of 5. The tool returns Variable. You can adjust any input and the result updates as you type — no submit button, no reload. That's the real power here: seeing how sensitive the output is to one or two assumptions.
What moves the number most
The result responds to Mortgage Balance, Fixed Rate, Current Variable Rate, and Fixed Period. Two inputs usually tip the answer one way or the other. Identify which ones matter most by flipping each value past a round threshold and watching whether the option with the lower calculated total changes.
The formula behind this
Simplified interest cost. Balance × rate × years. Lower cost = winner at current rates. Everything the calculator does is shown in the formula box below, so you can check the math against your own spreadsheet if you want.
What the headline rate hides
Lenders quote a rate; what you pay is a blend of that rate, fees, insurance, and any early-repayment penalty built into the product. The figure here isolates the core interest cost so you can compare like-for-like across deals — then add the other costs separately before signing anything.
What this doesn't capture
The figure excludes arrangement fees, valuation costs, legal fees, insurance, and any early-repayment charges — those can add several thousand to the headline cost. Rate changes at renewal for fixed-term deals will shift the picture further. Use this for the core interest/principal math and add the other costs on top.
Comparing a 5.5 fixed rate against 5 variable rate on £200,000 shows Variable in total costs.
Inputs
This example uses typical values for illustration. Adjust the inputs above to match a specific situation and see how the result changes.
Sources & Methodology
Methodology
Simplified interest cost. Balance × rate × years. Lower cost = winner at current rates.
Frequently Asked Questions
to choose fixed or variable?
How long to fix?
What about caps?
Future rate expectations?
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