How this calculator works
The calculator amortises your balance twice: once on the scheduled repayment, and once with your overpayments applied. Because every extra euro comes straight off the capital, less interest accrues on the smaller balance each month, so the mortgage clears sooner. The difference between the two runs is the interest and the time you save. A lump sum is treated as a single overpayment made now; a monthly overpayment is added to every repayment.
What overpaying does
Take a €300,000 balance at 3.9% with 25 years left, where the scheduled repayment is about €1,567 a month. Adding €200 a month clears the mortgage about 4 years and 4 months early and saves roughly €33,000 in interest. A single €20,000 lump sum today, with no monthly extra, saves about €30,000 and takes over two and a half years off the term. Small, regular overpayments and a one-off lump sum both work; the earlier they land, the more they save.
Overpaying on a fixed rate
Variable and tracker rates allow unlimited overpayments. On a fixed rate you have a penalty-free allowance each year, often around 10% of the balance but set differently by each lender, and going beyond it can trigger an early repayment charge. Ask your lender for your allowance and a charge quote before making a large overpayment on a fixed rate.
A note on accuracy
This tool assumes the overpayment shortens the term and that the rate holds for the remaining term. It does not model a lender's overpayment cap or any early repayment charge. For guidance on overpaying and early repayment, see the CCPC and Citizens Information, and our methodology for the formula.