Mortgage overpayment calculator
See how much interest overpaying your mortgage each month saves, how much sooner it is paid off, and your new mortgage-free date.
What overpaying saves
- Normal monthly payment
- Total interest if you never overpay
- Last payment if you never overpay
- Months to clear it with your overpayments
- Time saved
- Interest saved
- Mortgage-free date
Planning tool, not tax or financial advice. Assumes your interest rate stays the same for the whole term. Check your lender's overpayment rules before you pay extra.
Want the full month-by-month schedule and a remortgage check?
The HonestSums Mortgage Overpayment and Remortgage spreadsheet does everything above, plus:
- A full month-by-month schedule for up to 40 years
- One-off lump sums as well as regular overpayments
- A what-if table comparing £50, £100, £200, £300, £500 and £1,000 a month side by side
- An allowance check against your lender's yearly overpayment limit
- Your current deal against a new one, with product fees and early repayment charges
- A Checks tab that proves the maths against the standard mortgage formulas
Works in Excel and Google Sheets.
How mortgage overpayments save you money
Each month your lender charges interest on what you still owe: the balance times your yearly rate, divided by 12. Your normal payment covers that interest and pays off a little of the balance.
An overpayment goes straight off the balance. Next month there is less to charge interest on, so more of your normal payment clears the debt, and the effect builds every month after. With the same monthly payment, the mortgage ends sooner and you pay less interest in total.
Check two things with your lender
First, the overpayment limit. Many fixed-rate deals let you overpay up to 10% of the balance each year without an early repayment charge; go over it and you may pay a fee. Second, what the lender does with the money. Some shorten the term, some lower your monthly payment instead. This calculator assumes the term gets shorter, which saves the most interest.
What it assumes
A repayment mortgage, the same interest rate for the rest of the term, and the overpayment made every month alongside the normal payment. Real rates change when a fixed deal ends, so treat the result as a guide to the size of the saving.