Rule of 72 Calculator

It's a shortcut for estimating how long an investment takes to double at a fixed annual rate: divide 72 by the annual rate in percent. At 8% a year, money doubles in roughly 72 ÷ 8 = 9 years. You can also run it backwards to find the rate needed to double in a set number of years (72 ÷ years).

It's an approximation. The rule is most accurate for rates around 6–10%; outside that range it drifts from the true figure. This calculator shows the exact result alongside the rule — the exact doubling time uses t = ln(2) ÷ ln(1 + r), and the exact required rate uses r = 2^(1 ÷ t) − 1 — so you can see how close the shortcut is.

The mathematically exact constant for continuous compounding is about 69.3 (100 × ln 2). 72 is used instead because it's close enough for typical rates and divides cleanly by 2, 3, 4, 6, 8, 9, and 12 — which makes the mental arithmetic easy. Some people use 70 or 69.3 for more precision at low rates.

No. It assumes a single fixed annual rate and a one-time lump sum with no withdrawals, fees, taxes, or inflation. Real returns vary year to year, and inflation erodes purchasing power, so treat the result as a rough estimate rather than a guarantee.