Compound interest is often called the eighth wonder of the world because it lets your money earn money, which then earns money of its own. This calculator shows how an initial amount plus regular contributions can grow over time, and how powerfully small differences in rate, time, and compounding frequency change the outcome.
How compounding works
With compound interest, each period's interest is added to your balance, so the next period earns interest on a larger amount. The formula is A = P(1 + r/n)^(nt) for a lump sum, where P is principal, r is the annual rate, n is compounding periods per year, and t is years — with regular contributions added on top each period.
Why time is your greatest ally
The longer money compounds, the more dramatic the growth, because the largest gains happen in the final years when the balance is biggest. Starting early beats contributing more later — a saver who begins at 25 often ends up ahead of one who starts at 35 and contributes twice as much.
A worked example
Invest $10,000 at 8% for 30 years with no additions and it grows to about $100,600. Add $300 a month and the balance swells to roughly $540,000 — of which only about $118,000 is money you put in. The rest is compound growth doing the heavy lifting.
Making compounding work for you
- Start as early as you can — time matters more than the amount you invest.
- Reinvest all interest and dividends so they compound rather than sitting idle.
- Use the Rule of 72 (72 ÷ rate) to estimate how long it takes your money to double.
- Automate contributions so your balance grows steadily without relying on willpower.