Compound Interest & Rule of 72 Calculator

Anyone who wants to see, at a glance, what regular investing can become. Compound growth is hard to picture in your head — the chart makes the curve obvious, especially in the later years.

Compound interest means you earn returns on your returns, not just your original money. Early on the growth looks slow, but because each year builds on a bigger base, the curve bends sharply upward over long horizons — which is why starting early matters so much.

The calculator compounds monthly and adds your monthly contribution along the way, then plots the balance year by year.

The Rule of 72 is a mental-math shortcut: divide 72 by your annual return to estimate how many years it takes your money to double. At 8%, that's about 9 years; at 6%, about 12. It's an approximation, but a remarkably good one for typical rates.

Real returns aren't constant — markets rise and fall, and this assumes a steady rate. It also ignores inflation, taxes, and fees, which reduce real-world growth. Use it to understand the power of compounding and consistency, not as a precise forecast.