Free tool — instant result
Compound interest
calculer-facile.fr
Initial capital
€
Works with any currency — €, $, £, CHF…
Monthly contribution (optional)
/mo
Annual rate
%/yr
Duration
yrs
Fill in the fields to simulate growth.
How does compound interest work?
Formula: Final = C × (1 + r)ⁿ
Interest earns interest — the effect is exponential over time.
With simple interest, only the initial capital earns interest. With compound interest, interest is added to the capital and itself earns interest. Over 30 years at 5%: $10,000 → $25,000 (simple) vs $43,219 (compound).
Divide 72 by the annual rate to estimate how many years to double your money. At 6% per year: 72 ÷ 6 = 12 years. At 4%: 18 years. It's a quick mental shortcut.
With compound interest at 7% per year, $10,000 becomes $38,697 after 20 years. The interest earned is $28,697 — nearly 3× your initial investment.