Question 1

What is the interest formula?

Accepted Answer

The basic simple interest formula is I = P · r · t / 100, where I is the interest, P the principal, r the annual rate in percent and t the time in years. For daily interest use t = days / 360 (German method) or days / 365; for months, t = months / 12. Example: €10,000 at 3% for 90 days gives 10,000 · 3 · 90 / (100 · 360) = €75.

Question 2

How do I calculate daily interest?

Accepted Answer

Daily interest is I = P · r · days / (100 · 360) under the German method, or with 365 in the denominator under act/365. First count the interest days: under 30/360 every month has 30 days, act methods use actual calendar days. The deposit day usually does not count, the final day does. The calculator above counts the days for you from two dates.

Question 3

How do I rearrange the formula for principal, rate or time?

Accepted Answer

From I = P · r · t / 100 there are three rearrangements: principal P = I · 100 / (r · t), rate r = I · 100 / (P · t) and time t = I · 100 / (P · r). Time t comes out in years; multiply by 360 for interest days under the German method or by 12 for months. Just pick the unknown in the calculator — the worked solution shows each step.

Question 4

What is the difference between simple and compound interest?

Accepted Answer

With simple interest, interest is calculated only on the original principal and not reinvested. With compound interest, interest is added to the principal and earns interest the following year. Within one year the result is the same; over many years compound interest grows much faster. For multi-year investments with reinvestment, use our compound interest calculator.

Question 5

What is the German day-count method (30/360)?

Accepted Answer

Under the German method, also called commercial interest calculation, every month has 30 days and the year 360 days. The 31st of a month counts as the 30th, as does the last day of February. From 15 January to 15 April there are exactly 90 interest days. It is common at school and for savings books; banks often use act/360 or act/365 for overnight savings and loans.

Question 6

How high is German default interest in 2026?

Accepted Answer

German default interest follows § 288 BGB: base rate plus 5 percentage points if a consumer is involved, plus 9 points for transactions without a consumer. According to the Bundesbank the base rate has been 1.52% since 1 July 2026, giving 6.52% p.a. for consumers and 10.52% p.a. for businesses. In the first half of 2026 the base rate was 1.27%, i.e. 6.27% and 10.27%.

Question 7

When does the base rate change?

Accepted Answer

The base rate under § 247 BGB changes on 1 January and 1 July each year, by as many percentage points as the ECB main refinancing rate has moved since the last adjustment. The Bundesbank publishes the new value in the Federal Gazette. If a default period spans a change date, our calculator applies the rate valid for each section.

Question 8

Can I use the interest calculator for homework?

Accepted Answer

Yes. For every problem the calculator shows the basic formula, the rearrangement for the unknown, the inserted values and the result — just like in maths class. Set the method to 30/360 if your exercise uses 360 days a year, which is standard in German textbooks. Use it to check your own answer.

Guide: Interest Calculator

Simple Interest Explained: Formula, Daily Interest and Examples

Day-Count Methods Compared: 30/360, act/360, act/365 and act/act

Calculating German Default Interest in 2026: Base Rate, § 288 BGB and Examples

Calculating Interest on Savings and Fixed-Term Deposits

Simple Interest: 6 Exercises with Solutions

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