Rule of 72 (Doubling Time) Calculator
How long an investment takes to double at a given rate of return.
Every calculation runs locally in your browser. Nothing you enter here is sent to a server or stored anywhere.
Rule of 72 estimate
10.3 years
Exact doubling time
10.24 years
For informational and educational purposes only — not financial, investment, or tax advice, and not a substitute for consulting a qualified professional about your specific situation. TrueMeasureKit is not liable for decisions made based on these results. See our Terms of Service.
Where the Rule of 72 comes from
The exact doubling time is ln(2) / ln(1 + r), which isn't something most people can compute in their head. The Rule of 72 is a centuries-old shortcut: dividing 72 by the rate gives a close approximation, because 72 has a lot of small divisors (1, 2, 3, 4, 6, 8, 9, 12...) that match common interest rates well. It's most accurate in the 6–10% range and drifts further from exact at very high or very low rates — which is why this calculator shows both numbers side by side.
Frequently asked questions
What's the Rule of 72?
A quick mental-math approximation: divide 72 by the annual growth rate to estimate years to double. At 8% annual growth, that's roughly 9 years (72 ÷ 8) — this calculator gives the exact figure alongside the approximation.
How accurate is the Rule of 72 approximation?
It's quite accurate for rates between roughly 6% and 10%, and drifts more at very low or very high rates. This calculator computes the exact doubling time using logarithms, so you can see how far the quick estimate is off for your specific rate.