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.
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?
When is simple interest used?
Related calculators & reading
Sources
- Todos Contam: Portal de educação financeira — Banco de Portugal
Author: Thorben Rasmus Idel · Reviewed by: Nahar Geva · Last reviewed: 2026-05-31