Investment Calculators
Rule of 72.
How long does money take to double at a given rate? Get the exact answer next to the 72, 70 and 69.3 shortcuts, see how far each is off, or flip it around and find the rate needed to double in a set number of years.
Inputs
Result
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Worked example
At 8% a year, the exact doubling time is ln(2) / ln(1.08) = 9.006 years. The rule of 72 gives 72 / 8 = 9 years, the rule of 70 gives 8.75 years and the rule of 69.3 gives 8.66 years. At 8%, 72 is almost exactly right.
To go the other way: doubling in 10 years needs (21/10 − 1) = 7.177% a year exactly. The rule of 72 says 7.2%.
Formula notes
Exact: t = ln(2) / ln(1 + r), with r as a decimal rate per period and annual compounding. Shortcut: t ≈ N / r, with r in percent and N = 72, 70 or 69.3. Rate needed: r = 21/t − 1.
- 72 is used because it divides evenly by 1, 2, 3, 4, 6, 8, 9 and 12, and it works well at typical rates of about 6% to 10%.
- 70 fits better at lower rates (around 2%), and 69.3 is most accurate for continuous compounding.
- Inflation halving time: the same arithmetic tells you how long purchasing power takes to halve. At 3.5% inflation the rule of 70 gives 70 / 3.5 = 20 years. Enter an inflation rate in the rate box to get this figure.
- This page asserts no rates. Use whatever rate you want to test. Returns are never guaranteed, and fees and taxes are not included.
Source: Wikipedia, Rule of 72 (formula, rule variants, accuracy range and inflation use). As of 2026-09-30.
Frequently asked questions
How do I use the rule of 72?
Divide 72 by the annual percentage rate. At 6% that is 72 / 6 = 12 years to double.
How accurate is it?
It is a good approximation for annual compounding at roughly 6% to 10%, and less accurate at higher rates. The table above shows the error for your exact rate.
Does it work for debt or inflation?
Yes. Debt growing at a rate doubles on the same schedule, and the same division gives how long money takes to lose half its purchasing power.
Disclaimer
Educational only. This is arithmetic, not a forecast: real returns vary and are not guaranteed, and fees and taxes are not modelled.