Rule of 72: Estimate How Long Money Takes to Double
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Editorial verification: Examples on this page are checked against FutureCalc core formulas before publish. Source: cross-checked with OpenStax TVM basics and Investor.gov investing basics.
The Rule of 72 formula
Years to double ≈ 72 ÷ interest rate (%)
At 8%, 72 ÷ 8 ≈ 9 years. Confirm with the compound interest calculator or lump-sum growth tool.
72 vs 69.3 vs 70
Continuous compounding uses ln(2)≈0.693 → “Rule of 69.3”. For many annual rates between 6% and 10%, 72 stays surprisingly accurate. Outside that band, solve exactly with FV = PV × (1+r)n.
Worked example
$10,000 at 6%: Rule of 72 → 12 years. Exact annual compounding: n = ln(2)/ln(1.06) ≈ 11.9 years. Use the rate of return calculator when you know start/end values instead of the rate.