When an invoice is paid late
If a debtor does not pay an invoice that is due, they are in default. The creditor may then claim not only the outstanding amount but also default interest. German law sets the rate: § 288 BGB links it to the base rate published by the Deutsche Bundesbank. This article shows how to calculate default interest step by step, including when the base rate changes during the default period. In the interest calculator, simply select the calculation type "default interest".
When does default begin?
Default usually begins with a reminder after the due date. No reminder is needed if a calendar date was set for payment, such as "payable by 30 June". In addition, under § 286 (3) BGB a debtor is in default at the latest 30 days after the due date and receipt of the invoice. Towards consumers this 30-day rule only applies if the invoice expressly points it out.
Interest runs from the start of default until the day of payment. Whether and when default occurred in an individual case can be disputed. If in doubt, seek legal advice; this article is not legal advice.
The rate: base rate plus a surcharge
Default interest has two components:
- A consumer is involved (§ 288 (1) BGB): base rate + 5 percentage points.
- No consumer is involved (§ 288 (2) BGB), i.e. business-to-business: base rate + 9 percentage points.
The base rate under § 247 BGB is adjusted on 1 January and 1 July each year. Since 1 July 2026 it has been 1.52%; in the first half of 2026 it was 1.27%. This gives current rates of 6.52% p.a. for consumers and 10.52% p.a. for businesses. In the first half of 2026 the rates were 6.27% and 10.27%.
Between businesses a flat fee of €40 is added (§ 288 (5) BGB). It is credited against later legal costs, such as lawyer's fees.
The formula
Default interest is simple interest. Under § 289 BGB no further interest may be charged on default interest, so there is no compounding. In practice, actual calendar days and 365 days per year are usually used:
Default interest = claim · rate · days / (100 · 365)
Example 1: consumer, 45 days
A private customer pays a €1,200 tradesman's invoice 45 days late. The whole period lies after 1 July 2026, so the rate is 6.52%.
1,200 · 6.52 · 45 / (100 · 365) = €9.65
The amount is modest. With larger claims and longer periods it adds up quickly, though.
Example 2: business, across 1 July
A company pays another company's €8,000 invoice only on 15 August 2026; default began on 15 May 2026. The period crosses the 1 July change date, so it is split into two sections:
- 15 May to 1 July 2026: 47 days at 10.27% (1.27% + 9): 8,000 · 10.27 · 47 / 36,500 = €105.80
- 1 July to 15 August 2026: 45 days at 10.52% (1.52% + 9): 8,000 · 10.52 · 45 / 36,500 = €103.76
In total that is €209.55 default interest, plus the €40 flat fee. The interest calculator performs this split automatically when you enter a date range in default-interest mode, and shows each section in a table.
Common mistakes
- Wrong surcharge: for consumers it is 5, not 9 percentage points.
- Outdated base rate: the rate changes twice a year. Using an old value means claiming too much or too little.
- Compounding: default interest does not earn interest itself.
- Flat fee for consumers: the €40 only applies if the debtor is not a consumer.
Conclusion
Default interest is calculated with the simple interest formula once you know the right rate: base rate plus 5 or 9 percentage points. If the base rate changes during the default, calculate each section separately. The basics of the formula are covered in simple interest explained.