How long until your money doubles? Divide 72 by your return — or flip it around to find the return you need.
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| Rate | Rule of 72 | Exact time |
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The Rule of 72 is the fastest mental shortcut in investing: divide 72 by your annual interest rate to estimate how many years it takes for money to double. At 8%, your money doubles in about 9 years. At 12%, in about 6 years. No spreadsheet required.
Years to double ≈ 72 ÷ Annual interest rate
Reversed: Rate needed ≈ 72 ÷ Years you have. Want to double your money in 6 years? You need roughly a 12% annual return (72 ÷ 6).
The mathematically exact constant is 69.3 (100 × ln 2), but 72 won because it divides evenly by 2, 3, 4, 6, 8, 9 and 12 — the rates people actually encounter. That divisibility makes it a mental-math tool; 69.3 is a calculator tool.
Nearly perfect for rates between 6% and 10% — at 8%, the rule says 9 years and the exact answer is 9.01 years. It drifts at extremes: at 2% the rule says 36 years vs an exact 35, and at 20% it says 3.6 vs 3.8. The table above shows both side by side so you can see the gap yourself.
The rule assumes a single lump sum compounding untouched. It doesn't account for regular contributions (that's where a SIP calculator helps), taxes, fees or inflation. Use it for quick comparisons; use the full calculators for real plans.
Divide 72 by your annual interest rate to estimate the years needed to double your money. At 8%, that's about 9 years.
Very accurate for 6–10% rates (within months of exact). It drifts at very low or high rates — check the exact column in the table above.
72 ÷ 8 = 9 years. The exact logarithmic answer is 9.01 years.
69.3 is the exact constant for continuous compounding, but 72 divides evenly by common rates, making it the practical mental-math choice.
Run exact compounding with monthly contributions and charts.
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