Estimate years to double money or annual return needed using Rule of 72.
Updated
Approx years to double: 6.00
Approx annual return needed: 12.00%
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In-depth guide
The Rule of 72 is a mental-maths shortcut: divide 72 by an annual return rate and you get a close estimate of how many years it takes an investment to double. At 8% a year, money doubles in roughly 72 ÷ 8 = 9 years. This tool runs the calculation both ways in your browser.
True doubling time under compound interest is ln(2) ÷ ln(1 + r), and ln(2) ≈ 0.693, or 69.3%. 72 is used instead because it is close to that figure and divides cleanly by 2, 3, 4, 6, 8, 9 and 12 — making the mental arithmetic easy. The approximation is most accurate for rates in the 6–10% range.
For high rates some people switch to 70 or 69.3 for accuracy; for low rates 72 stays close. It also ignores inflation, taxes and fees, which all stretch the real time to double your purchasing power.
The calculation runs entirely in your browser — the figures you enter are never uploaded.
Use this tool for quick planning, comparison, and what-if finance scenarios. Use official calculators, a qualified adviser, or source documents before filing taxes, signing contracts, or making irreversible money decisions.
A mental shortcut: divide 72 by the annual return rate to estimate how many years it takes an investment to double.
No. It is an approximation that is most accurate for rates roughly in the 6 to 10 percent range.
Yes. Provide a target number of years and the tool estimates the annual return you would need.
The mathematically exact figure is about 69.3, but 72 is close and divides cleanly by many numbers, making the mental arithmetic easy.
No. It assumes a steady compounding return before costs, so real time to double your purchasing power is longer.
Some people use 70 or 69.3 for lower rates or when more accuracy matters; 72 stays convenient for quick mental estimates.
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