Rule of 72 Calculator
How long it takes to double your money — the classic mental-math shortcut, plus the exact answer
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Quick summary
Divide 72 by your annual return to estimate years to double your money: at 8% that's 9 years, at 6% it's 12 years. It's a shortcut for the exact compound-interest formula (ln(2) ÷ ln(1 + rate)) — accurate within a few percent for typical investment returns (4–15%), drifting more at very high rates. Run it backwards to find the rate needed to double your money in a target number of years.
Reviewed by Simple Calculators TeamLast reviewed
For planning only — not financial or tax advice. Results are estimates based on your inputs and the stated assumptions. Confirm important decisions with a qualified professional.
How to use this calculator
- Enter the inputs requested by the calculator.
- Choose the units or options that match your situation and review the assumptions.
- Run or update the calculation to see the result.
- Review the result, breakdown, and formulas shown on the page.
- Check the inputs, units, dates, and limitations before saving or sharing the result.
Frequently asked questions
How does the Rule of 72 work?
72 divided by the annual percentage return approximates the number of years to double an investment, because 72 has many convenient divisors (2, 3, 4, 6, 8, 9, 12) matching common rates. It comes from a linear approximation of the exact logarithmic compound-growth formula, which is why it's closest to exact in the 6–10% range.
How accurate is the Rule of 72?
Very close for rates between 4% and 15% — typically within a few weeks of the exact answer. It drifts more outside that band: at 2% it slightly underestimates the time, and at 30%+ it noticeably overestimates. For anything beyond quick mental math, use the exact-answer figure this calculator also shows.
How long does it take to double money at 7%?
About 10.3 years by the rule (72 ÷ 7), versus 10.24 years exactly — a very close match. Historically, the US stock market has averaged roughly 10% nominal returns long-term, implying a doubling time near 7.2 years before inflation.
What rate do I need to double my money in 10 years?
About 7.2% by the rule (72 ÷ 10), or 7.18% exactly. Use rate-to-double mode to check any target timeline — useful for reverse-engineering how aggressive an investment needs to be to hit a savings goal by a specific date.
Key terms
- Rule of 72
- 72 ÷ annual rate ≈ years to double — a mental-math approximation of compound growth.
- Compound Growth
- Growth where each period's return applies to the prior period's total, not just the original principal.
- Doubling Time
- Years for an investment to reach 2× its starting value at a constant rate.
Methodology, sources & limitations
This page shows the formula, assumptions, and limitations used for this estimate. Rates, thresholds, and other time-sensitive inputs can change, so verify them against the relevant official source before relying on a result.
For our review approach and the primary sources we consult, see the methodology and source policy. If you spot an outdated figure or formula, please contact us with a correction.