Rule of 72 Calculator
The Rule of 72 is a fast way to estimate how long money takes to double at a fixed annual return. Enter a rate to see the years, or enter a number of years to see the rate, with the exact figure shown alongside.
How to use the Rule of 72 calculator
- Pick a mode: solve for years to double, or for the return rate needed.
- Enter your annual return percent, or the number of years you have.
- Compare the quick Rule of 72 estimate with the exact compounding result.
Examples
8% return, time to double
rate = 8%
Rule of 72: 9 years (exact 9.01 years)
Double in 9 years, rate needed
years = 9
Rule of 72: 8% (exact 8.01%)
Frequently asked questions
What is the Rule of 72?
It is a shortcut for estimating how long an investment takes to double. Divide 72 by the annual return percent and the result is roughly the number of years. You can also divide 72 by the years to get the rate needed.
Why 72 and not another number?
The true doubling math uses the natural logarithm of 2 (about 0.693). Multiplied out it gives roughly 69.3, but 72 is close to that and divides cleanly by 2, 3, 4, 6, 8, 9 and 12, which makes the mental math easy.
How accurate is the Rule of 72 versus the exact formula?
Very close for everyday rates. Around 6 to 10 percent the estimate is within a fraction of a year. The gap widens at very low or very high rates, where 72 over the rate drifts further from the exact logarithmic answer.
What about the Rule of 70 or 69.3?
Both are alternatives to 72. The Rule of 69.3 is the most exact for continuous compounding, and the Rule of 70 is a simple round number. The Rule of 72 trades a little precision for far easier division, which is why it is the most popular.
Does the calculator account for fees, taxes or inflation?
No. It uses a single nominal annual return. Fees, taxes and inflation all slow real doubling, so treat the result as a clean illustration rather than a guarantee.
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