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Interest Calculator: Simple & Daily Interest

Solve for interest, principal, rate or time using the formula I = P · r · t / 100 — with day-count conventions, German statutory default interest (§ 288 BGB) and a step-by-step solution you can follow.

100% freeNo data storedBase rate as of 1 Jul 2026

Simple or compound interest?

This calculator covers simple interest: interest is not reinvested. For multi-year investments where interest earns interest, use the compound interest calculator.

What do you want to calculate?

Values

€
%

Period & day-count method

Every month has 30 days, the year 360 — common for savings books and at school.

Your interest

€75.00

€10,000.00 · 3 % p. a. · 90 days

Principal + interest

€10,075.00

Time in years (t)

0.25

≈ 3 months

Interest per day

€0.83

Step-by-step solution

  1. 1

    Basic simple interest formula

    I = P · r · t / 100

  2. 2

    Determine time t in years

    t = 90 / 360 = 0.25

  3. 3

    Insert values and calculate

    I = 10,000 · 3 · 0.25 / 100

  4. 4

    Result

    I = €75.00

Compared with compound interest

With simple interest, capital grows linearly; with compound interest, exponentially. Under one year there is no difference — interest only earns interest from the second year on.

Simple interest

€75.00

Compound (annual)

€75.00

Difference

€0.00

Simple vs. compound interest over 10 years

Go to compound interest calculator

Assumptions & notes

  • Simple interest: interest is not added to the principal and does not earn interest itself. Formula: I = P · r · t / 100 with t in years.
  • With dates, the first day is not counted, the last day is (end minus start). With 30/360 (German method), the 31st and the last day of February count as the 30th.
  • With months or years, t = months / 12 or t = years regardless of method. Interest days are derived from that using the method's year basis.
  • German base rate under § 247 BGB per Deutsche Bundesbank: 1.52% since 1 July 2026 (as of 24 Sept 2026). This calculation is non-binding and not legal advice.

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Guide: Understanding Interest Calculation

Interest formula, daily interest, day-count methods and default interest explained

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Frequently Asked Questions

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.

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.

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.

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.

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.

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%.

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.

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.