Compound Interest Calculator
Compound interest is interest that earns interest on the interest already accrued. By reinvesting what you earn, your money grows faster over time. Use the calculator to see how much you could accumulate from an initial amount plus monthly contributions.
Year-by-year breakdown
| Year | Interest | Balance |
|---|---|---|
| 1 | €79 | €2,279 |
| 2 | €224 | €3,624 |
| 3 | €437 | €5,037 |
| 4 | €722 | €6,522 |
| 5 | €1,084 | €8,084 |
| 6 | €1,525 | €9,725 |
| 7 | €2,051 | €11,451 |
| 8 | €2,665 | €13,265 |
| 9 | €3,371 | €15,171 |
| 10 | €4,175 | €17,175 |
Educational estimate, not financial advice. Returns are not guaranteed.
Video: how to use the calculator
What compound interest is
Unlike simple interest (which is earned only on the initial capital), compound interest is also earned on the interest already credited. Each period the balance grows, and so does the base the next interest is calculated on.
The formula
With monthly compounding and regular contributions: FV = P·(1+i)^n + PMT·((1+i)^n − 1)/i, where i is the annual rate divided by 12 and n is the number of months. The calculator applies this month by month.
What moves the result most
Time is the most powerful factor: the earlier you start, the more compounding cycles occur. The interest rate and how regularly you contribute come next.
Worked example
With €1,000 to start, €100 per month, at a 5% annual rate over 10 years, you invest €13,000 in total and end with about €17,175: over €4,000 comes from compound interest alone.
Frequently asked questions
What is the difference between simple and compound interest?
How often is interest compounded?
Are the results guaranteed?
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Sources
- Todos Contam: Portal de educação financeira — Banco de Portugal
Author: Thorben Rasmus Idel · Reviewed by: Nahar Geva · Last reviewed: 2026-05-31