Mortgage overpayment calculator
Overpaying a repayment mortgage cuts both its length and its interest, because every extra pound comes straight off the balance that interest is charged on. Enter what you owe, your rate and the years left, then add a monthly overpayment or a lump sum to see how much sooner it ends and what you save.
CoreFi does not advise on or arrange mortgages. This is arithmetic from the figures you enter, not a recommendation. Check your lender's overpayment allowance and early repayment charges before you overpay.
Worked example, illustrative only
Owing £200,000 at 4.5% with 25 years left, the payment is £1,112 a month. Overpaying £200 a month ends the mortgage 6 years 1 month sooner and saves £36,280 of interest, with the rate held where it is. Illustrative only, not a quote or advice.
How it works
- Each month's interest is the rate over twelve on what you owe. An overpayment reduces what you owe straight away, so every later month charges interest on less.
- Lenders usually keep the monthly payment the same, so an overpayment shortens the term. Some lower the payment instead unless you ask; check which yours does.
- Most fixed and discounted deals let you overpay up to 10% of the balance a year without a charge. Above that, an early repayment charge, often 1% to 5%, is usually charged on the excess.
- Once the deal ends, overpayments are normally free. The figures hold one rate for the whole term, which real mortgages rarely keep.
Overpay or save?
An overpayment earns, in effect, your mortgage rate: it is interest you will not pay, with no tax on it. Money in savings earns its own rate, may be taxed, and stays within reach, which an overpayment usually does not. So the comparison is your mortgage rate against a savings rate after tax, and how likely you are to need the money. The cash vs investing calculator shows what savings keep after inflation.
Frequently asked questions
How much can I overpay without a charge?
Most fixed and discounted deals allow up to 10% of the outstanding balance each year without an early repayment charge. Some measure it at the start of the deal year, some at the start of the calendar year, and some let unused allowance carry over. Your mortgage offer sets out the exact terms, so check it before overpaying.
What is an early repayment charge?
A charge for repaying more than the allowance during a fixed or discounted deal, usually a percentage of the amount repaid that falls each year, for example 5%, 4%, 3%, 2% and 1% over a five-year fix. This calculator estimates it on the amount above the allowance, which is the common arrangement.
Is a lump sum or a monthly overpayment better?
A pound paid earlier saves more interest than a pound paid later, so a lump sum today saves more than the same total spread across a year. Regular overpayments are easier to fit into a budget and to keep inside the yearly allowance.
Will overpaying lower my monthly payment?
It depends on the lender. Many keep the payment the same and shorten the term, which is what this calculator shows. Others recalculate a lower payment over the original term, which saves less interest. You can usually choose, so ask your lender.
Should I overpay my mortgage or save?
This calculator does not recommend either. Overpaying saves interest at your mortgage rate; savings earn their own rate after tax and stay available. Many people keep an emergency fund before overpaying. Compare the two rates and how easily you might need the money.
Is this advice?
No. It is arithmetic from the figures you enter, not advice, a recommendation or a quote. CoreFi does not advise on or arrange mortgages. Your lender's own figures, including its allowance and charges, are the ones that count.
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This calculator illustrates arithmetic from the figures you enter. It is not advice, a recommendation or a quote. CoreFi does not advise on or arrange mortgages. CoreFi is a trading name of JG Core Ltd (company 16218779). Figures reviewed September 2026.