How this calculator works
Each month your lender charges interest on what you still owe: balance × annual rate ÷ 12. Your payment first covers that interest, and the rest reduces the balance. An overpayment goes straight to the balance, so next month's interest is lower and more of your regular payment goes on the debt. That compounding effect is why a modest overpayment can take years off a mortgage.
This calculator keeps your regular payment the same and uses overpayments to shorten the term. When the rate changes (for example when a fixed deal ends), the regular payment is recalculated on the balance you still owe over the remaining original term, as lenders normally do, and your overpayment carries on top.
For interest-only mortgages the regular payment covers interest only, so the balance doesn't fall unless you overpay, and whatever is left is due at the end of the term. Part and part splits the loan: one portion is repaid over the term, the other is interest-only.
Dates assume your next payment is due next month and that the balance and term you enter are what's left today.
Limits of the estimate: lenders may calculate interest daily or annually rather than monthly, so their figures can differ by a small amount. The calculator doesn't include fees, early repayment charges, or changes to your payment dates.
Frequently asked questions
How much interest will I save by overpaying my mortgage?
It depends on your balance, rate, term and how early you start. Overpayments made early have the most effect, because they cut the balance that interest is charged on for the longest. Enter your own figures above to see the saving in pounds and the time taken off the term.
Will overpaying shorten my mortgage term or lower my monthly payment?
It varies by lender. Many shorten the term and keep your payment the same, which is what this calculator assumes. Some will recalculate and lower your payment instead, so check with your lender or look at your mortgage terms.
Is there a limit on how much I can overpay?
Many fixed and discounted deals let you overpay up to a set amount each year without a charge, often 10% of the balance, and may charge an early repayment charge (ERC) on anything above that. Some deals, including many trackers and standard variable rate mortgages, have no limit. The rules differ by lender and product, so check yours before overpaying a large amount.
What happens to my payments when my fixed rate ends?
When a deal ends you usually move to your lender's standard variable rate unless you switch to a new deal (remortgage). Your payment is then recalculated at the new rate. The "If rates are higher" table above shows how your payment could change if the new rate is 1, 2 or 3 percentage points higher than you assume.
Is overpaying always better than saving the money?
Not necessarily. It depends on your mortgage rate compared with what savings would earn, whether you'd still have an emergency fund, any other more expensive debts, and the charges on your mortgage. This calculator shows the effect of overpaying; it can't tell you whether it's right for you. For personal advice, speak to a regulated mortgage adviser.
What is an interest-only mortgage?
You pay only the interest each month, so the amount you borrowed isn't reduced. You need a plan to repay the full balance when the term ends, such as savings, investments or selling the property. A part-and-part mortgage repays some of the loan over the term and leaves the rest interest-only.