Mortgage Overpayment Calculator

Why overpaying your mortgage works

A mortgage charges interest on the balance you still owe. Every pound you overpay comes straight off that balance, so from then on there is less debt for interest to be charged on — and that saving compounds month after month for the rest of the term. Because the effect builds over many years, even a modest regular overpayment can shave several years off the loan and save thousands of pounds. The earlier in the mortgage you start, the bigger the effect, since the saved interest has the longest time to add up.

How the calculator works

The tool runs two amortisation schedules side by side. The first follows your existing mortgage: each month it adds interest at your rate, then subtracts your normal payment. The second does the same but adds your overpayment on top of every monthly payment, so the balance falls faster. It then compares how long each takes to reach zero and totals the interest paid in each case. The difference between the two is your interest saved, and the difference in months is your time saved.

Monthly overpayments vs a lump sum

Both approaches save money, but in different ways. A lump sum — say a bonus or inheritance — cuts the balance in one go and immediately reduces the interest charged from that point on. Regular monthly overpayments are easier to sustain and let the saving build gradually. The golden rule is that overpaying sooner always beats overpaying later, because the saved interest has more time to compound. If you can do both, a lump sum early plus steady monthly top-ups is the most powerful combination.

Check your overpayment limits first

Most fixed-rate mortgages allow you to overpay up to 10% of the balance each year penalty-free. Go above that during a fixed deal and you may trigger an early repayment charge, which can be 1–5% of the amount and could wipe out your saving. Always check your mortgage terms or ask your lender before making large overpayments. The free, government-backed service MoneyHelper explains overpayment rules and has its own calculator, and Citizens Advice can help if you are unsure.

Should you overpay or save?

Before overpaying, weigh it against other priorities. Keep an emergency fund of three to six months' expenses, and clear any higher-interest debt such as credit cards first, since those usually cost far more than a mortgage. After that, compare your mortgage rate with the interest you could earn on savings: if the mortgage rate is higher, overpaying generally gives the better guaranteed return. Overpayments are also tax-free, whereas savings interest may not be — another point in favour of clearing the mortgage when rates are similar.