How compound interest works
Compound interest means you earn interest on both your original money and the interest it has already earned. Over time, that reinvestment makes your balance grow faster and faster — this is why it's often described as "earning interest on interest."
This calculator assumes monthly compounding and contributions added at the end of each month.
The rule of 72
A quick way to estimate how long money takes to double is to divide 72 by your annual return. At a 7% return, for example, money roughly doubles about every 10 years (72 ÷ 7 ≈ 10.3).
Why starting early matters
Because growth accelerates over time, the early years matter enormously. A smaller amount invested in your 20s can end up worth more than a larger amount started in your 40s — the difference is time in the market, not timing the market.