What an extra €100 a month actually does to your mortgage

An extra mortgage payment can bring the payoff date forward and reduce future interest. To estimate the effect, compare two schedules for the same balance and rate: one with the regular payment, and one with the extra amount applied to principal.

For the example below, an extra €100 a month clears a €250,000 loan about four years and eight months earlier and saves roughly €58,860 in interest. Those figures depend on the assumptions; they do not tell you whether overpaying is the best use of your money.

What the regular payment covers

Assume a €250,000 loan, a fixed nominal annual interest rate of 6.5%, and 360 monthly payments over 30 years. Interest is calculated monthly at 6.5% divided by twelve. There are no fees, rate changes or payment breaks.

The regular principal-and-interest payment is about €1,580.17. The first payment covers:

• €1,354.17 of interest on the opening balance;
• €226.00 of principal, reducing the amount owed.

Interest takes a large share of this particular payment because the balance and rate are substantial relative to the payment. A lower-rate or shorter-term loan can have a different split from the start.

How the extra €100 helps

If the lender applies the extra €100 directly to principal, the first month’s principal repayment becomes about €326.00. Next month’s balance is €100 lower than it would otherwise have been, saving about €0.54 in that month’s interest alone.

Keep the regular payment unchanged and the interest saving leaves more of it available to repay principal. Repeating that process brings the final payment forward. The benefit is avoided future interest, not an extra €100 of profit each month.

Compare the full schedules

These results assume an extra payment every month from the first payment onwards. The calculation carries unrounded balances and reduces the last payment to the amount still due. A lender rounding each month to cents may produce slightly different totals.

Extra per month	Payoff time	Total interest	Interest saved
€0	30 yr 0 mo	€318,861	—
€50	27 yr 5 mo	€285,950	€32,911
€100	25 yr 4 mo	€260,001	€58,860
€200	22 yr 1 mo	€221,243	€97,618
€500	16 yr 3 mo	€155,345	€163,516

For the €100 case, there are 303 payments of about €1,680.17, followed by a smaller final payment of about €909.81. It would be wrong to multiply €1,680.17 by all 304 months and call that the total paid.

The extra payments are earlier repayment of money already owed. Comparing their sum with interest saved is not a conventional investment-return calculation: the cash flows occur at different times.

Check how your lender handles overpayments

Ask whether an extra payment reduces principal immediately and whether it shortens the term or lowers later payments. The table assumes a shorter term with the normal payment maintained. A lender that recalculates the payment can give a different result.

Also check overpayment allowances, early repayment charges and any instructions needed to mark a payment as principal-only. An amount used to cover fees or advance a future instalment does not necessarily have the effect modelled here.

For an existing mortgage, start with the current balance and remaining term, not the original loan amount and original term. If a fixed-rate deal ends before the mortgage does, a constant-rate projection is only one scenario.

Earlier payments and the wider decision

Under the same rate and payment assumptions, reducing principal earlier leaves less balance generating interest for longer. That explains the timing effect without making early overpayment a universal recommendation.

Access to savings, other debts, taxes, repayment charges and uncertainty about alternative returns all affect the decision. “Overpay whenever the mortgage rate exceeds the savings rate” leaves out those considerations.

Read the calculator’s schedule as well as its headline

The loan calculator shows monthly principal, interest and balance, with a CSV download for checking individual payments. Compare the baseline with the extra-payment schedule and inspect the smaller final instalment.

Its fee-based Estimated APR is not a verified lender APR/APRC: the current calculation assumes equal payments even when the final one is smaller, and its rate solver can be unreliable. The interest examples here use the monthly schedule, not that APR estimate. Use the lender’s own disclosure when comparing actual borrowing costs.

Last reviewed 2 October 2026.