What is the interest formula?

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.

How do I calculate daily interest?

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.

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

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.

What is the difference between simple and compound interest?

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.

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

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.

How high is German default interest in 2026?

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%.

When does the base rate change?

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.

Can I use the interest calculator for homework?

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.

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

Why there are several day-count methods

The interest formula I = P · r · t / 100 is unambiguous as long as time is given in whole years. As soon as days are involved, a question arises with surprisingly many answers: how many days does a month have, and how many a year? Day-count conventions answer exactly that. Depending on the method, the same deposit earns slightly different interest.

In the interest calculator you can choose all four common methods and compare the results directly.

The four methods at a glance

30/360: the German method

Every month has 30 days, the year 360. The 31st of a month is treated as the 30th, as is the last day of February. It is also called commercial interest calculation. It is convenient because months convert neatly: three months are always 90 interest days, half a year always 180. That is why it is standard in German textbooks and used for many savings books and savings contracts.

act/360: the money-market method

Actual calendar days are counted, but divided by 360. Because a real year has 365 days, a full year under act/360 yields slightly more than the nominal rate: 365 / 360 = 1.0139, so 3% effectively becomes about 3.04%. The method is common in money markets and for many overnight and fixed-term deposits in the euro area.

act/365: the English method

Actual days divided by 365, even in leap years. A full year therefore equals the nominal rate, except in a leap year, where 366 / 365 applies. The method is common in the UK and is often used in Germany for default interest under § 288 BGB.

act/act: exact days

Actual days divided by the actual length of the year, i.e. 365 or 366. If the period spans a year end, each calendar year is calculated separately. This is the most precise method and is standard for bonds, for example when calculating accrued interest.

Worked example: €10,000 at 3% from 1 February to 1 August 2026

The period covers 181 actual days (February 28, March 31, April 30, May 31, June 30, July 31) and 180 interest days under the German method.

30/360: 10,000 · 3 · 180 / 36,000 = €150.00
act/360: 10,000 · 3 · 181 / 36,000 = €150.83
act/365: 10,000 · 3 · 181 / 36,500 = €148.77
act/act: 2026 is not a leap year, so same as act/365 = €148.77

The range is just over €2. With larger amounts or longer terms it grows accordingly: at €100,000 it would be about €20.

Where the differences show

Month end and February

From 31 January to 28 February 2026 there are 28 actual days. Under the German method there are 30 interest days, because both dates count as the 30th. In 31-day months it is the other way round: from 1 to 31 March, act counts 30 days and the German method only 29, because the 31st counts as the 30th. These effects even out over a year but can be noticeable for short periods.

Leap years

In the leap year 2028, February has 29 days. act/365 still divides by 365, act/act by 366. A whole leap year under act/365 therefore yields 366 / 365 of the annual interest, under act/act exactly the annual interest.

Full years

For a full calendar year, 30/360 and act/act give exactly the nominal rate. act/360 gives slightly more, as does act/365 in a leap year.

Which method should I choose?

School exercises: 30/360 unless the exercise says otherwise.
Overnight and fixed-term deposits: see the terms and conditions, often act/360 or act/365.
German default interest: in practice usually act/365.
Bonds: usually act/act.
Private loans: freely agreed, simplest are 30/360 or act/365.

If you want to check what your bank credited, look at the terms and conditions. Set the method stated there in the calculator and the result should match to the cent.

Conclusion

The day-count method decides how days become a fraction of a year. At school 30/360 is standard; banks and courts usually count actual days. The differences are small but measurable. Read more about the interest formula and how to rearrange it in simple interest explained.

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