Rule of 72 Calculator

The Rule of 72 is a quick mental-math shortcut: divide 72 by an annual interest rate to estimate how many years it takes an investment to double. Enter a rate below to see both the quick estimate and the exact answer.

Rule of 72 Calculator

Live

Annual interest rate (%)

Years to double (Rule of 72)

12.0 yrs

Exact doubling time

11.9 yrs

Difference

0.1 yrs

Most accurate for rates roughly between 6% and 10%.

The formulas

Rule of 72 estimate: Years ≈ 72 ÷ Rate Exact doubling time: Years = ln(2) ÷ ln(1 + Rate ÷ 100)

Example

At 6% annual return: 72 ÷ 6 = 12.0 years (Rule of 72 estimate), versus the exact answer of about 11.9 years — a difference of just 0.1 years, showing how close the shortcut gets for typical rates.

Step-by-step guide

Enter an annual interest or growth rate.
Read the Rule of 72 estimate for a quick mental-math answer.
Compare it to the exact doubling time to see how accurate the shortcut is at that particular rate.

Common mistakes

Using the Rule of 72 for very low or very high rates — the shortcut is most accurate roughly between 6% and 10%, and drifts further from the exact answer outside that range.

Assuming a steady, guaranteed rate — the Rule of 72 assumes constant compounding, while real investments fluctuate year to year.

Frequently asked questions

Why 72 specifically?

72 has many small divisors (1, 2, 3, 4, 6, 8, 9, 12...), making the mental division easy for common rates, and it happens to approximate the exact logarithmic formula well in the typical 6–10% range.

Can I use this for debt instead of investments?

Yes — the same shortcut estimates how quickly an unpaid balance would double at a given interest rate, which is a sobering way to look at high-interest credit card debt.

Is there a "Rule of 114" or "Rule of 144"?

Yes — some people use 114 to estimate tripling time and 144 to estimate quadrupling time, using the same divide-by-rate shortcut.

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