Rule of 72 Calculator
Estimate how many years it takes an investment to double.
Input sheet
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The Rule of 72 is the back-of-the-envelope trick every investor knows: divide 72 by your annual return and you have a quick estimate of how many years it takes your money to double.
How it works
Divide 72 by the annual rate of return to estimate the years for money to double. It's an approximation of the exact logarithmic formula, accurate for typical single-digit rates.
It works because the exact doubling-time formula involves a logarithm, and 72 happens to approximate that result closely for the mid-single-digit returns most investments deliver. It is an estimate, not an exact answer, but it's accurate to within a fraction of a year across the 6–10% range.
The same idea scales: divide 114 by the rate to estimate tripling time, or 144 to estimate quadrupling. This tool also shows the Rule of 70 variant, which some prefer for continuous compounding.
Years to double ≈ 72 ÷ annual return percent
Worked examples
An investment returning 8% per year → ≈ 9.0 years to double
72 ÷ 8 = 9.0 years. The exact figure is about 9.0 years, so the rule is essentially spot-on here.
An investment returning 6% per year → ≈ 12.0 years to double
72 ÷ 6 = 12.0 years, and quadrupling takes roughly twice that, about 24 years.
Tips & gotchas
- Use it as a sanity check, not a planning figure — for precise growth, use the compound interest calculator with your exact rate and horizon.
- The rule drifts at extreme rates: above ~20% it overstates doubling time, and below ~4% it slightly understates it. Stay in the single-to-low-double-digit range for best accuracy.
- Flip it to find the rate you need: to double in 6 years, you need roughly 72 ÷ 6 = 12% annually.
- Apply it to inflation too — at 3% inflation, prices double in about 24 years, which is why a 'safe' cash hoard quietly loses half its purchasing power over a working career.
FAQ
Why 72?
72 has many small divisors and closely matches the exact doubling time for the 6–10% returns most investments fall in. Some use 70 or 69.3 for continuous compounding.
How accurate is the Rule of 72?
Very accurate for typical returns of 6–10%, usually within a couple weeks of the exact figure. It loses precision at very high or very low rates.
Should I use 72 or 70?
Use 72 for annually compounded returns — it divides cleanly by many rates. Use 70 (or 69.3) for continuous compounding, where it's marginally more precise.
Can I use it for debt?
Yes. At a 22% credit card APR, 72 ÷ 22 ≈ 3.3 years for an unpaid balance to double — a stark illustration of why high-interest debt is so dangerous.
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