Rule of 72 Calculator
Do the classic mental-math trick for doubling money — divide 72 by your rate for the years, or by the years for the rate you'd need.
How it works
The Rule of 72 is a back-of-the-napkin shortcut: divide 72 by your annual return and you get roughly how many years it takes to double your money. At 8% a year, 72 ÷ 8 lands on about 9 years. Flip it around and 72 ÷ years tells you the rate you'd need to double in a set time.
It's an approximation, and the calculator shows the exact compounded figure right beside it so you can see how close the shortcut gets. For rates in the everyday 6% to 10% range, the Rule of 72 is impressively accurate — usually within a few months of the real answer.
Why it's handy: you can run it in your head. Hear that an account earns 6% and you instantly know money doubles in about 12 years. It turns abstract percentages into something concrete, which makes it easier to judge whether a return is actually worth getting excited about.
Frequently asked questions
Why 72 and not another number?
72 is used because it divides cleanly by lots of common rates — 2, 3, 4, 6, 8, 9, 12 — and it happens to match real compound growth closely for typical interest rates. Purists sometimes use 69.3 for continuous compounding, but 72 is friendlier for mental math.
How accurate is the Rule of 72?
Very close for rates between about 6% and 10%, where it's usually within a fraction of a year. It drifts a bit at very high or very low rates, which is why this tool also shows the exact compounded result for comparison.
Can I use it to find the rate I need?
Yes. Switch to 'Rate needed' and enter how many years you want your money to double in. Divide 72 by that number and you get the approximate annual return required — double in 9 years needs roughly 8% a year.