ZeroServer.tools

Rule of 72 Calculator

Calculate how long it takes to double your investment using the Rule of 72, 69, and 70. Enter your annual return rate to see the comparison.

Annual Return Rate (%)

Principal Amount ($)

Doubling Time Comparison at 8% annual return

Method	Formula	Years to Double
Rule of 72	72 ÷ r	9.00
Rule of 69	69.3 ÷ r	8.66
Rule of 70	70 ÷ r	8.75
Exact (ln 2)	ln(2) ÷ ln(1 + r/100)	9.0065

$10,000

Starting Amount

$20,000

Doubled Value (in ~9.0 yrs)

Years to Double at Common Rates

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

About the Rule of 72 Calculator

The Rule of 72 is a mental math shortcut: divide 72 by the annual return rate to estimate how many years it takes to double an investment with compound interest. For example, at 8% return: 72 ÷ 8 = 9 years. The Rule of 69.3 is more accurate for continuous compounding, while the Rule of 70 is common in economics for GDP and inflation doubling. The exact formula uses the natural logarithm: ln(2) ÷ ln(1 + r/100).

Private & free — this tool runs entirely in your browser.