Micro Tools

Rule of 72 calculator

Enter an annual return rate to see how many years it takes your money to double, rule-of-thumb and exact.

Inputs

Results update live as you type.

Annual return rate

Years to double

Enter values to calculate.

Exact doubling time

Rule estimate

Formulas: years = 72 / rate. Exact: ln(2) / ln(1 + rate). The rule is closest for rates between 6 and 10%.

Why 72 works

Doubling time math involves logarithms, but 72 divided by the rate approximates it closely for typical returns because 72 has many divisors (2, 3, 4, 6, 8, 9, 12). At 8%, 72/8 = 9 years vs 9.01 exact.

Using it in reverse

Flip it to find the rate you need: rate = 72 / years. Want to double money in 6 years? You need about 12% annual returns. It also exposes inflation: at 3%, prices double in 24 years.

Rule of 72 Calc FAQs

What is the Rule of 72?

Divide 72 by your annual return rate to estimate years to double your money. At 8%, money doubles in about 9 years.

How accurate is the Rule of 72?

Very close for rates of 6 to 10% (within a few percent of exact). It drifts at extremes: at 2% it says 36 years vs 35 exact, still usable.

Can I use it for debt too?

Yes, and it is sobering: credit card debt at 24% APR doubles in 3 years (72/24) if you pay nothing. The rule works for anything compounding.

Rule of 72 vs Rule of 70?

70 is slightly better for low rates near 2 to 4%, 72 better for 6 to 10% and easier mental math. Both are approximations of the same logarithm.

Is my data uploaded anywhere?

No. Every calculation happens in your browser with JavaScript. Nothing you enter leaves your device.

Inputs

Why 72 works

Using it in reverse

Related tools

Rule of 72 Calc FAQs