Rule of 72 Calculator
The Rule of 72 is the classic shortcut for compound growth: divide 72 by your return to see how many years it takes your money to double. This calculator runs it both ways — rate to doubling time, or doubling time to required rate.
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The Rule of 72 is an approximation - the exact compounding figures below show the precise value for comparison. Estimates only, not investment advice.
How to use this calculator
Pick a mode. In "Rate → doubling time", enter your annual return and starting amount; in "Time → required rate", enter how many years you want the doubling to take. Results update instantly.
Worked example: at 8%, the Rule of 72 says your money doubles in 9.0 years — so $10,000 becomes $20,000 in 9 years. Flip to the other mode and enter 9 years: the calculator returns the matching 8.00% rate.
How it works
Doubling time = 72 ÷ annual rate. Required rate = 72 ÷ years. The final amount after one doubling is simply 2 × the starting amount.
For comparison, the calculator also shows the exact math: doubling time = ln(2) ÷ ln(1 + r), and required rate = 2^(1 ÷ years) − 1. At 8% the rule gives 9.0 years versus 9.006 exact — the shortcut is an approximation, and works best for rates between 6% and 10%.
Frequently asked questions
What is the Rule of 72?
A quick mental-math shortcut: divide 72 by your annual interest rate to get roughly how many years it takes money to double. At 8%, money doubles in about 9 years.
Is the Rule of 72 accurate?
Very close. At 8% the rule says 9.0 years while exact compounding says 9.006 years - the difference is negligible for planning.
When does the Rule of 72 not work well?
It is an approximation that works best for rates between 6% and 10%. At very low or very high rates the error grows, so use the exact compounding figure shown in the results.
What about the Rule of 70 or Rule of 69.3?
Variations tuned for continuous compounding (69.3) or easier division (70). For everyday rates, 72 divides into more whole numbers, which is why it became the standard.