Smart Math guide · Everyday Math

Use the Rule of 72 as an Estimate

For moderate percentage growth rates, 72 divided by the annual rate gives a rough doubling time.

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The method

How to use it

Use the rate as a percent number, not decimal.
Compute 72/rate.
Treat it as an approximation and compare with exact compound growth when accuracy matters.

Worked example

At 8% growth, doubling time ≈72/8=9 years.

Check your thinking

Exact annual compounding doubles in about 9.0 years.

Watch out

The rule is approximate and less accurate at very high or unusual rates.