Rule of 72 Calculator
Quickly estimate how long it takes for an investment to double. Years to double ≈ 72 ÷ annual rate. Works in reverse too — given a doubling time, what rate is needed.
Inputs
Used only for the reverse calculation.
Years to double
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Rate needed for target doubling time
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Exact doubling time (compound)
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About Rule of 72 Calculator
How the rule works
It’s a shortcut for compound growth: at small rates, ln(2) / ln(1 + r) ≈ 0.72 / r. The "72" makes it easy mental math. Most accurate around 6-10% rates.
When it’s less accurate
At very high rates (> 15%) the rule slightly underestimates; at very low rates (< 4%) it slightly overestimates. The exact figure is shown for comparison.
Practical use
Use it to compare investments quickly. 8% doubles in ~9 years; 6% takes ~12; 12% takes only ~6. Inflation works the same way in reverse — 3% inflation halves purchasing power in ~24 years.