Investment Calculators

Simple Interest Calculator

Simple interest is always earned only on the initial capital: the base never changes. Use the calculator to see how much it earns, and compare side by side with the effect of compound interest.

Initial amount€Annual interest rate (%)%Years

Future value

€1,500

Interest earned

€500

With compound interest: €1,647 (+€147)

Educational estimate, not financial advice. Returns are not guaranteed.

Video: how to use the calculator

What simple interest is

With simple interest, interest is calculated on the initial capital and is not added to the base. Growth is therefore linear: the same amount of interest each period.

The formula

Interest = Principal × rate × time. The future value is the principal plus the interest. The rate is annual and time is in years (may be fractional).

Simple vs compound interest

Unlike compound interest, simple interest does not earn on the interest already accrued. Over long horizons the difference becomes significant: the calculator shows both.

Worked example

With €1,000 at 5% a year for 10 years, simple interest earns €500 (total €1,500). With compound interest, the same capital would reach about €1,647: the difference is "interest on interest".

Frequently asked questions

What is the difference between simple and compound interest?

Simple interest is always earned only on the initial capital; compound interest is also earned on accumulated interest, growing faster over time.

When is simple interest used?

It is common in some short-term loans and in teaching examples. Most savings products use compounding (compound interest).

Related calculators & reading

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<p style="font:13px/1.5 system-ui,-apple-system,sans-serif;margin:8px 0 0"><a href="https://www.calculadoracapital.pt/en/calculators/simple-interest">Simple Interest Calculator</a> by <a href="https://www.calculadoracapital.pt">Calculadora Capital</a></p>
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Sources

Todos Contam: Portal de educação financeira · Banco de Portugal

Author: Thorben Rasmus Idel · Reviewed by: Nahar Geva · Last reviewed: 31 May 2026