Rule of 72 calculator
The rule of 72 is a quick mental shortcut: divide 72 by the rate to get the rough years to double.
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How to use it
- Enter what do you know? and value (rate % or years).
- The result appears straight away and updates as you edit.
- Copy the result, or copy a link that keeps the same values to share it.
How it is calculated
Estimate: years = 72 / rate %. Exact with annual compounding: years = ln 2 / ln(1 + r). Reverse: rate ≈ 72 / years, exact = 2^(1/years) − 1.
Example
Inputs: What do you know? = The annual rate, find the years, Value (rate % or years) = 8. Result: Years to double, rule of 72: 9 years; Exact years: 9.0065 years; Years, rule of 70: 8.75 years.
Limits and caveats
- The shortcut is close between about 6% and 10% and drifts at very high rates.
- The exact value assumes annual compounding at a fixed rate.
Questions about this tool
Why 72?
It sits close to 100 × ln 2 (about 69.3), adjusted to suit common rates, and divides evenly by many numbers.
What is the rule of 70?
A variant that suits lower rates and is often used for inflation or economic growth.
Does it work for inflation?
Yes. Divide 72 by the inflation rate to see roughly when your purchasing power halves.