What overpaying actually does
Your normal mortgage payment is calculated to clear the loan exactly at the end of the term, so most of it, especially in the early years, is swallowed by interest. An overpayment is different: every extra pound bypasses the interest bill and attacks the balance directly. A smaller balance means less interest next month, which means more of your normal payment also hits the balance, and the effect snowballs from there.
This calculator simulates your mortgage month by month. It works out your normal payment from the balance, rate and remaining term, then replays the mortgage with the overpayment added, counting how many months earlier you finish and how much interest never gets charged. The final overpaying month is usually a smaller, partial payment that clears whatever balance remains.
Worked examples
Watch the early repayment charge
Most fixed-rate deals let you overpay up to 10% of the outstanding balance in each year of the deal without penalty. On a £200,000 balance that is £20,000 a year, far more than most people's spare cash, so a monthly overpayment rarely breaks the limit. Larger lump sums can, though, and the charge for exceeding the allowance is typically 1% to 5% of the excess, often stepping down each year of the fix. Check your mortgage offer before making a big payment, and note that some lenders measure the 10% against the balance at the start of each deal year rather than the calendar year. If you are near the end of a fix it can pay to wait until the deal ends, when most mortgages allow unlimited overpayment.
Overpay or do something else with the money?
Overpaying is a guaranteed, tax-free return equal to your mortgage rate, which is hard to beat for a no-risk use of spare cash. But it is not always first in the queue. Expensive debt such as credit cards or overdrafts should go first, and an emergency fund of a few months' outgoings matters more than an earlier mortgage-free date, because overpaid money is hard to get back out of a house. If your savings rate beats your mortgage rate after tax, saving can also win on pure numbers. Figures here assume your rate never changes, which no real mortgage manages for 25 years, so treat the results as a comparison of scenarios rather than a promise. This is guidance, not financial advice: for impartial help see the government's Money and Pensions Service or a qualified adviser.
Frequently asked questions
How much can I overpay without a penalty?
Most fixed and discounted deals allow overpayments of up to 10% of the outstanding balance each year without charge. Go beyond that and an early repayment charge applies, typically 1% to 5% of the excess. Check your mortgage offer for the exact allowance, and note that many trackers and standard variable rates have no limit at all.
Why does a small overpayment save so much?
Every extra pound goes straight at the balance, and interest is charged on the balance every month for the rest of the term. Killing £200 of balance today avoids interest on that £200 for what could be 20 years or more, which is why the savings compound into thousands.
Should I reduce the term or reduce the payment?
When you overpay, lenders usually offer to either shorten the term or lower your future monthly payment. Shortening the term saves far more interest, because the debt is cleared sooner. Lowering the payment adds flexibility but keeps you borrowing for just as long.
Is it better to overpay or put the money in savings?
Compare rates. If your mortgage rate is higher than the after-tax return on your savings, overpaying usually wins. If savings pay more, or you lack an emergency fund of 3 to 6 months of outgoings, building cash first is often the wiser move. Clearing expensive debts like credit cards should come before either.
Does a one-off lump sum work the same way?
Yes, and often better, because the whole amount starts saving interest immediately. A lump sum early in the mortgage saves more than the same total spread over years of small payments. The same 10% annual allowance normally applies during a fixed deal.