Late payment interest calculator (viivis)

Example: a 2,500 EUR invoice due 15 July and paid 14 September

The debt is 2,500 EUR, the due date 15.07.2026 and the invoice was paid on 14.09.2026. The rate is the statutory 10.40% per year that applies from 1 July 2026.

Days first. Interest starts running on the day after the due date, which is 16 July. July contributes 16 days (16th to 31st), August 31 and September 14, for a total of 61 days. The calculator arrives at the same number by subtracting the due date from the payment date.

Then the amount: 2,500 × 0.104 × 61 ÷ 365 = 43.45 EUR. Interest per day is 2,500 × 0.104 ÷ 365 = about 0.7123 EUR, shown rounded as 0.71 EUR. Multiplying the rounded daily figure by the days would give 43.31 EUR, which is why the calculator works out the total from the exact daily rate. The debt including interest is 2,543.45 EUR.

How days are counted, and small edge cases

The day count is the most common source of errors when working out late payment interest, especially when days are counted by hand from a calendar. Check that the count starts on the day after the due date.

• If the invoice is paid on the due date, there are zero days overdue and no interest.
• One day late: a 480 EUR invoice due 01.08 and paid 02.08 gives 1 day and 0.14 EUR of interest. With small sums and short delays the interest is often a matter of cents.
• The payment day counts as a day overdue; the due date itself does not. That is why 1 June to 8 July is 37 days, not 38.
• The calculator always divides the annual rate by 365 days, including in a leap year.

When the debt spans a half-year change

The statutory rate changes on 1 January and 1 July. In the first half of 2026 it was 10.15%, in the second half it is 10.40%. The calculator applies one rate to the whole period, so for a longer delay it is worth calculating each half-year separately.

Example: a 12,000 EUR debt due 31.03.2026 and paid 30.09.2026, 183 days in total. First run: due 31.03, paid 30.06, rate 10.15%. That gives 91 days and 303.67 EUR. Second run: due 30.06, paid 30.09, rate 10.40%. That gives 92 days and 314.56 EUR. Total interest is 618.23 EUR.

Running all 183 days at 10.40% gives 625.71 EUR, 7.48 EUR too much. At 10.15% it gives 610.67 EUR, 7.56 EUR too little. On a small debt the gap is negligible, but with a large amount and several half-years it adds up.

Contractual interest stated per day

Contracts often state late payment interest as a percentage per day. The calculator asks for an annual rate, so multiply the daily rate by 365: 0.1% per day is 36.5% per year and 0.05% per day is 18.25% per year.

Example: an 800 EUR debt due 10.05.2026 and paid 09.07.2026, with contractual interest of 0.1% per day. Enter 36.5 as the rate. The result is 60 days, 0.80 EUR per day and 48 EUR in total. Check: 800 × 0.001 × 60 = 48 EUR.

A second example: a 3,000 EUR invoice due 01.09.2026 and paid 30.09.2026, interest 0.05% per day. A rate of 18.25% gives 29 days and 43.50 EUR of interest, 1.50 EUR per day. The limits for consumer contracts are covered in the FAQ below.

Partial payments, and what the calculator leaves out

If the debt was paid in instalments, calculate each period separately using the amount that was unpaid at the time. Example: 2,000 EUR due 01.08.2026, 1,000 EUR paid on 31.08 and the remaining 1,000 EUR on 30.09, rate 10.40%. The first period is 30 days on 2,000 EUR, or 17.10 EUR; the second is 30 days on 1,000 EUR, or 8.55 EUR; 25.65 EUR in total. This is a simplification: how a partial payment is split between the principal and interest already accrued depends on the contract and the law.

The calculator works out late payment interest only. Collection costs, contractual penalties, court costs and other claims have to be added separately. The result is a calculated figure based on the data entered and does not replace checking the contract terms, or legal advice if there is a dispute.