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Rule of 72 Calculator.

Estimate how long an investment takes to double at a constant positive annual rate.

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Results update as you type.

Rule-of-72 estimate: 9 years

Rule-of-72 estimate

0.00years
Exact constant-rate result: 9.01 years

Exact constant-rate result

0.00years
The exact comparison assumes one constant annual effective rate and no fees, taxes, deposits, or withdrawals.

FAQs

Is the Rule of 72 exact?

No. It is a quick approximation; actual doubling time depends on compounding frequency, changing returns, fees, and taxes.

Use Cases

Quick mental math for doubling time

Use the Rule of 72 to get a fast, rough estimate of how long it takes to double your money at a given annual return, without needing a calculator.

Example: At 8% return, 72 ÷ 8 = 9 years (exact is about 9.01 years).

Compare investment scenarios

Enter different annual return rates to see how the doubling time changes, helping you compare potential investments or savings goals.

Example: Compare 6% vs. 10%: 12 years vs. 7.2 years (exact: 11.9 vs. 7.27).

Frequently Asked Questions

How does the Rule of 72 calculator work?
Enter your annual return (as a percentage). The calculator divides 72 by that rate to estimate the number of years to double. It also shows the exact result using compound growth, so you can see how close the rule is.
Why is the Rule of 72 an estimate?
The rule simplifies the math by using 72, which works well for rates between about 6% and 10%. For very low or high rates, the estimate becomes less accurate, so the calculator also provides the exact compound-growth figure.
Can I use this for any investment?
Yes, as long as the annual return is constant and positive. It's a general tool for any investment that compounds annually, such as stocks, bonds, or savings accounts. It does not account for taxes, fees, or variable returns.

Tips & Common Mistakes

Tips

  • For rates between 6% and 10%, the Rule of 72 is very accurate; outside that range, rely on the exact result shown.
  • Use the exact compound-growth result for precise planning, especially for long-term investments.
  • Remember that this assumes a constant annual return; real investments fluctuate, so treat the result as an estimate.
  • To estimate tripling time, you can use the Rule of 114, but this calculator focuses on doubling.

Common Mistakes to Avoid

  • Entering the return as a decimal (e.g., 0.08) instead of a percentage (8). The calculator expects the percentage value.
  • Assuming the Rule of 72 is exact for all rates; it's an approximation, so always check the exact result for accuracy.
  • Using the estimate for non-compounding or variable-return investments without adjusting expectations.

Last updated: August 13, 2026