How mortgage overpayments save you interest

A mortgage overpayment is any amount you pay above the scheduled repayment, and every euro of it goes straight at the balance. That euro stops generating interest for the rest of the loan, because interest is charged on what you still owe, not on what you originally borrowed. On a €300,000 mortgage at 4% over 30 years, paying €100 extra a month removes €28,747 of interest and clears the loan 3 years and 6 months early.

Why the effect is so large

A mortgage is amortisation in action: each repayment splits into interest on the outstanding balance and a bit of principal, and the interest share shrinks slowly as the balance falls. In year one of the €300,000 example, €17,187 is repaid but €11,904 of it — 69% — is interest. Only €5,283 actually reduces what you owe.

An overpaid euro skips that slow grind entirely. It reduces the balance immediately, which means the lender charges interest on a smaller sum every single month afterwards, for as long as the loan would otherwise have run. A lower balance this month keeps the balance lower next month, and the month after that, so the saving compounds over the decades the loan has left to run. It is compound interest working in reverse: instead of a balance growing because interest keeps adding to it, the balance shrinks and takes future interest charges down with it, permanently.

The numbers

Two overpayment levels on the same €300,000 mortgage at 4% over 30 years, where the unchanged schedule would otherwise run the full term and cost €215,609 in total interest:

Extra payment per month	Interest saved	Time saved	Loan cleared in
€100	€28,747	3 years 6 months	26 years 6 months
€200	€50,412	6 years 2 months	23 years 10 months

Doubling the overpayment more than doubles the saving, because the larger overpayment also compounds against a shrinking balance for longer before the loan ends.

Earlier is stronger

The balance is at its largest at the very start of the mortgage, so an overpayment made in year one retires the most expensive euros in the loan — the ones that would otherwise have carried interest for the full remaining term. The same €100 a month made late in the loan, once the balance has already fallen a long way, saves far less, because there are fewer years of interest left for it to cancel. After 10 years the balance in this example stands at €236,352; after 20 years it is down to €141,463. An overpayment made against that smaller, older balance simply has less interest left to remove.

This does not mean late overpayments are pointless — they still work, and they still shorten the loan — but the earlier the euro arrives, the harder it works.

Check before you start

Overpaying is a feature of the mortgage, not a right, and the terms of your own contract are the final word. Variable-rate mortgages usually allow unlimited overpayments with no penalty. Fixed-rate mortgages usually cap how much you can overpay each year, or charge a break fee if you exceed it, because the lender has locked in funding at that rate and an unexpected early repayment disrupts its own arithmetic. Check your letter of offer or ask your lender before committing to a plan.

Beyond that check, the mechanics do not care how the extra money arrives. A regular €100 or €200 added to every monthly payment and an occasional lump sum dropped in when you can afford it reduce the balance in exactly the same way, euro for euro. What matters is the total amount and how early it lands, not the pattern.

Overpay or save instead?

Overpaying earns a guaranteed return equal to your mortgage rate, because every euro you pay early is a euro of interest at that rate you will never be charged. A savings account typically pays a lower rate, and that interest is usually taxed, so the comparison often favours overpaying on the numbers alone.

But the two are not equivalent in one important way: money overpaid into a mortgage is locked into the house. You cannot call it back next month if the car breaks down or the boiler fails. An accessible emergency fund should come before any overpayment plan, whatever the rate difference looks like on paper. Beyond that, the right split between overpaying and saving depends on your own job security, other debts, and plans for the money — a decision for a person and their own circumstances, not something a website can settle for you.