A recurring extra payment or a one-off lump sum both go straight against principal, so every euro overpaid stops accruing interest for the rest of the term. This calculator runs your actual repayment schedule month by month, with and without the overpayment, and shows the real interest saved and time shaved off.
A worked example
Take a €300,000 mortgage at 4% over 30 years. The standard monthly repayment is €1,432.25, and over the full 360 months you would pay about €215,609 in interest. Now add a recurring €200 a month on top of the repayment. Run through the same month-by-month schedule, the mortgage is cleared after 286 months instead of 360, which is 74 months (6 years and 2 months) earlier. Total interest falls to about €165,197, a saving of roughly €50,412.
A one-off lump sum behaves differently. Paying €10,000 at the end of year one on the same mortgage saves about €20,764 in interest and shortens the term by 21 months. The two plans put in different totals, so they are not a like-for-like comparison, but they show the same effect: money that reduces the balance in year one stops accruing interest for the other 29 years.
Why timing matters more than size
Interest is charged on the outstanding balance each month, so an overpayment saves interest for every remaining month of the term. The same €10,000 paid in year one saves far more than €10,000 paid in year twenty. That is why the calculator works month by month and asks in which year a lump sum lands, rather than estimating annually.
What the calculator assumes
- A fixed interest rate for the whole term, with equal monthly repayments.
- Overpayments go entirely against principal and are accepted without penalty.
- Your repayment stays the same, so the mortgage ends sooner rather than the payment shrinking. Some lenders may instead offer to reduce the monthly payment, which saves less interest but eases cash flow.
If your rate changes, or you are on a fixed deal with an overpayment cap, treat the result as an upper bound and check your lender's terms.
Frequently asked
Does my lender let me overpay for free?
Most Irish variable-rate mortgages allow overpayments with no penalty, but many fixed-rate deals cap penalty-free overpayments (commonly around 10% of the balance a year) or charge a breakage fee above that. Check your own mortgage terms before overpaying — this calculator assumes the whole overpayment is applied penalty-free.
Try it with your own numbers. Free, runs entirely in your browser — nothing you enter is sent anywhere.
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