Mortgage Overpayment Calculator

Why overpayments reduce interest so dramatically

Mortgage interest is calculated on the outstanding balance each month. When you overpay, the extra amount goes directly to principal, which means the balance shrinks faster. A lower balance means less interest charged the following month, which frees up more of your regular payment to go to principal — a compounding effect that snowballs over time.

The earlier in the mortgage term you start overpaying, the greater the impact. In the first years of a repayment mortgage most of the regular payment goes to interest rather than principal, so reducing the balance at this stage cuts interest over a much longer remaining period.

The overpayment formula

The standard amortisation formula gives the regular monthly payment. The overpayment calculator then re-runs the amortisation schedule with an increased monthly payment to find the new payoff month and total interest, and subtracts the two totals.

Early repayment charges and lender limits

Most mortgage lenders allow a fixed percentage of the outstanding balance to be overpaid each year without penalty — commonly 10% in the UK and varying widely in the US depending on the loan type and state. Exceeding this allowance can trigger an early repayment charge (ERC), which may partially or fully offset the interest saving.

Always check your mortgage terms or contact your lender before committing to a regular overpayment. If you are in a fixed-rate period, ERCs are most common. Tracker and variable-rate mortgages may permit unlimited overpayments.

Overpayment versus other uses of spare cash

Overpaying a mortgage is essentially a guaranteed return equal to the mortgage interest rate, because every pound or dollar applied to principal no longer accrues interest. Compare this rate to the after-tax return available on savings accounts, ISAs, 401(k) contributions, or investment accounts before deciding how to allocate surplus income.

A common rule of thumb is to prioritise higher-rate debt (credit cards, personal loans) before overpaying a mortgage, then compare the mortgage rate against accessible savings rates. If your savings rate exceeds the mortgage rate after tax, saving may generate a better outcome than overpaying.