Mortgage overpayment calculator
What overpaying really saves in interest — and how many years it cuts.
Most fixed-rate deals let you overpay around 10% of the balance each year penalty-free; beyond that, early-repayment charges (often 1–5%) can wipe out the gain — check your offer document first. Illustration only, not financial advice — assumes a constant rate for the whole term.
Every pound you overpay comes straight off the loan balance, so it stops accruing interest for every remaining year of the term. That compounding is why modest overpayments punch above their weight: £100 a month extra on a £200,000 mortgage at 4.5% over 25 years clears the loan roughly three and a half years early and saves around £21,000 in interest. A one-off lump sum early in the term works the same way — the earlier it lands, the more years of interest it kills.
Most fixed deals let you overpay up to 10% of the outstanding balance each year without penalty; go beyond that and an early repayment charge (typically 1–5% of the excess) applies. Check your own deal’s allowance before setting up a standing order — it is in the mortgage offer document. Once a fix ends and you are on a variable rate, there is usually no limit.
Whether overpaying is the best use of the money is a genuine question, not a foregone conclusion. Overpaying earns you a guaranteed, tax-free return equal to your mortgage rate — hard to beat for a basic-rate taxpayer when savings rates are below it. But money overpaid is hard to get back, so build an accessible emergency fund first, and clear any debt charging more than the mortgage (credit cards, car finance) before touching the mortgage at all. Pension contributions, with employer matching and tax relief, often beat overpaying too. Illustration, not advice.
Common questions
Is it worth overpaying my mortgage by £100 a month?
On a £200,000 loan at 4.5% over 25 years, £100 a month extra saves roughly £21,000 in interest and clears the mortgage about 3½ years early. On a smaller or cheaper loan the absolute saving is smaller but the principle holds: the return equals your mortgage rate, guaranteed and tax-free.
How much can I overpay without a penalty?
Most fixed and tracker deals allow 10% of the outstanding balance per year penalty-free; a few allow 20% or are unlimited. Beyond the allowance, early repayment charges of 1–5% of the excess apply, usually stepping down through the fix. On a lender’s standard variable rate there is normally no limit.
Should I overpay or reduce my monthly payment?
When you overpay, lenders ask whether to shorten the term or reduce future payments. Shortening the term saves far more interest — reducing the payment mostly recycles your overpayment into slightly cheaper months. Pick “reduce term” unless you specifically want lower committed outgoings.
Should I overpay the mortgage or put money in savings?
Compare rates after tax: if your mortgage charges 4.5% and accessible savings pay less than that after any tax, overpaying wins mathematically. But savings stay accessible and overpayments generally don’t — so keep 3–6 months of essential spending in cash first, and remember pensions (employer match, tax relief) often beat both.
Numbers are half the story. Check the home itself.
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