Rule of 72 Calculator

Estimate how long it takes to double your money by dividing 72 by the interest rate, and compare it with the exact answer and future value. Example: at 7% your money doubles in about 10.3 years by the rule, or 10.24 years exactly.

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Annual return

Starting amount (optional)

Years to grow (optional)

How to use the Rule of 72 Calculator

Enter the annual interest rate or return.
Optionally add a starting amount and years to see the future value.
Press Calculate to compare the rule with the exact answer.

Formula and method

Years to double is about 72 / rate (in percent). The exact figure is ln(2) / ln(1 + rate). Future value = starting amount x (1 + rate)^years.

Worked example: 7% a year

Step	Result
72 / 7	10.29 years
Exact	10.24 years
$10,000 after 20 years	$38,696.84

Doubling time by rate

Rate	Rule of 72	Exact
2%	36.0 years	35.00 years
4%	18.0 years	17.67 years
6%	12.0 years	11.90 years
8%	9.0 years	9.01 years
10%	7.2 years	7.27 years
12%	6.0 years	6.12 years

Why 72

72 has many small divisors, which makes mental math easy. The rule also works for inflation: at 3% inflation prices double in about 24 years.

Frequently asked questions

What is the rule of 72?

A shortcut: divide 72 by the annual rate to estimate years to double.

How accurate is it?

Very close from about 6% to 10%.

Does it work for inflation?

Yes, it shows how fast prices double.

What is the rule of 114?

The same idea for tripling: 114 divided by the rate.

What if the rate is 1%?

The rule says 72 years, exact is about 69.7.

Is the return guaranteed?

No. Investment returns vary.

Sources

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Effortless Math Team

5 minutes ago