Rule of 72 Calculator
Estimate how long money takes to double (or the rate needed) with the Rule of 72.
Best for: Use it to sanity-check investment growth, compare rates quickly, or estimate how fast inflation erodes purchasing power (72 / inflation rate = years for prices to double).
Purpose: Estimate how long money takes to double (or the rate needed) with the Rule of 72.
Input
Result
EstimateAt 8.00% per year, money doubles in about 9.0 years.
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- Type
- Industry Standard
- Method
- Industry-standard method
- Confidence
- High
Uses the standard formula and conventions the industry relies on.
The result is an estimate, not an exact figure. It is closest for rates around 6-10%; outside that range it drifts slightly from the precise value given by ln(2) / ln(1 + rate).
- Inflation often matters more than expected over the long run.
- Small rate changes can significantly affect total outcomes.
- Long-term consistency usually beats short-term timing.
Small, consistent changes compound: time in the market usually beats timing the market.
How it's calculated & sources
Free & no sign-up · runs entirely in your browser. Results are estimates for general information, not professional advice — verify important decisions with a qualified expert. Last reviewed June 2026.
How it works
Years to double ≈ 72 ÷ rate(%). Rearranged: rate ≈ 72 ÷ years. It is a mental-math shortcut; the exact answer uses logarithms.
Example
At 8% a year, money doubles in about 72 ÷ 8 = 9 years. To double in 6 years you need about 72 ÷ 6 = 12%.
Frequently asked questions
How accurate is the Rule of 72?+
It is very close for rates between about 6% and 10%. For lower or higher rates it drifts a little from the exact figure (which uses natural logarithms); some people use 70 or 69.3 for continuous compounding.
Can I use it for inflation?+
Yes — dividing 72 by an inflation rate estimates how many years until prices double and your money's purchasing power halves.
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