Thinking of leaving your fixed-rate mortgage early? Enter your current deal and a new rate to see whether the interest savings outweigh the Early Repayment Charge — with a clear break-even verdict.
7 min readUpdated: 25 Jul 2026UK
Leaving a fixed-rate mortgage early usually triggers an Early Repayment Charge, typically 1–5% of the balance — often several thousand pounds. Whether switching is worth it depends on how much you would save at today's rates versus that charge. This calculator works out your break-even month and gives a clear verdict on ditching your fix.
When mortgage rates fall, borrowers still locked into a higher fixed deal face a tempting question: is it worth paying the Early Repayment Charge to jump onto a cheaper rate now? The answer hinges on three numbers — how much lower the new rate is, how long is left on your current fix, and the total cost of switching (the ERC plus valuation and legal fees).
This calculator settles it by finding your break-even point. It amortises both your current mortgage and the new deal — adding the switching costs onto the new balance, as most lenders do — while keeping your monthly payment the same. Each month it tallies the interest you'd save on the cheaper rate. The month that running total overtakes your switching cost is the moment the switch has paid for itself.
Reading the verdict
If you break even comfortably before your current fix ends, switching is usually worth serious consideration — and because you keep paying the same amount, the lower rate also shortens your term. If break-even lands after the ERC period, or the numbers never recover the cost, the calculator flags it clearly. Either way, treat this as a starting point and confirm the details with an FCA-regulated mortgage adviser before acting.
Your mortgage is secured debt and runs on its own schedule. For unsecured debts — credit cards, loans and overdrafts — see our debt snowball method guide and the rest of our debt payoff guides.
Frequently Asked Questions
An ERC is a fee your lender charges if you leave a fixed-rate deal before it ends. It's usually a percentage of your outstanding balance — often 1–5%, typically stepping down each year. On a £200,000 balance a 2% ERC is £4,000.
It depends on how much lower the new rate is, how much time is left on your fix, and total switching cost. This calculator finds your break-even point — where cumulative interest savings exceed the switching cost. Break even well before your fix ends and it's usually worth considering.
The tool amortises both your current mortgage and the new deal (with switching costs added) using your existing monthly payment, comparing the interest paid each month. When the running total of interest saved exceeds the total switching cost, you've broken even.
When your fixed-rate deal ends you are automatically moved onto your lender's Standard Variable Rate (SVR), which is usually much higher than the fix and can change at any time. To avoid the jump you can remortgage to a new deal or take a product transfer with your existing lender, ideally lined up before the fix expires.
Start looking around six months before your fixed rate ends. A mortgage offer is typically valid for three to six months, so you can lock in a new deal early and have it ready to complete the day your current fix expires — avoiding any time on the lender's Standard Variable Rate. There is usually no obligation to proceed if rates improve.
Yes. This calculator is indicative only and uses generalised amortisation rules. Your exact ERC, product fees and eligibility depend on your lender. Always speak to a qualified, FCA-regulated mortgage adviser before deciding.
Disclaimer: This calculator is indicative only and based on generalised amortisation rules. It does not constitute financial or mortgage advice. Your actual ERC, fees and eligibility depend on your lender and circumstances. Always consult a qualified, FCA-regulated mortgage adviser before making any decision about your mortgage.
How this calculator works›
The method
It compares two futures for the rest of your current fixed period: staying on your existing rate, versus paying the Early Repayment Charge (ERC) now and switching to a new rate today. For each option it amortizes your mortgage month by month — interest on the balance plus your monthly payment — and totals the cost, including the ERC and any new-deal fees. The break-even month is the point where switching becomes cheaper than staying; before it, you are still recovering the ERC.
Assumptions & limitations
It uses the balance, rates, remaining fixed term and charges you enter; your exact ERC and rate come from your own mortgage illustration (ESIS).
It assumes each rate holds for the compared period and does not predict future Bank of England base-rate moves.
It excludes valuation, legal and broker costs unless you include them in the fees you enter.
Data sources
Every calculation runs in your browser on the balances and rates you enter, so the
results reflect your own figures.
Where we cite external data we use MoneyHelper — remortgaging and Bank of England — effective interest rates.
Figures and rules change over time — always check the original source.
Written and reviewed by Peter Barclay,
a UK Chartered Mechanical Engineer — who builds and maintains these tools, pairing
engineering-mathematics training with a focus on the mechanics of debt repayment. The calculators use standard amortization formulas and fixed
repayment orders. Read our methodology
or more about the author.
DebtSnowball is not a financial adviser and is not authorised or regulated by
the FCA (Financial Conduct Authority).
These tools are for education and information only, not financial advice.
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DebtSnowball.co.uk provides free educational information and calculators, not regulated financial advice. Calculator results are estimates for illustrative purposes only, based on the figures you enter, and should not be treated as financial advice. For advice tailored to your situation, speak to a free debt-advice charity above or an adviser regulated by the FCA. This site is supported by advertising; ads do not influence our editorial content.