How to Use the Rule of 72 Doubling Time Calculator
The "Rule of 72" is a famous shortcut for estimating how many years it takes for your money to double at a given average annual return — just divide 72 by the return rate. No calculator or logarithm table required; if you know your return, you can mentally estimate the doubling time almost instantly, which is why investors have relied on it for generations.
This calculator shows the Rule of 72 approximation side by side with the exact result from the logarithmic formula — ln(2) divided by ln(1 + rate) — so you can see exactly how accurate the shortcut is for your numbers. Enter a principal amount as well, and it will show you the projected value once your money has doubled.
Between roughly 6% and 10% returns, the two results are nearly identical. At very low rates (under 2%) or very high rates (over 20%), the gap between the approximation and the exact figure widens somewhat. For precise financial planning, lean on the exact logarithmic result rather than the rule-of-thumb approximation.
Frequently Asked Questions
72 approximates the logarithmic compounding formula and happens to divide evenly by many common integer return rates (around 6-12%), making it easy to calculate mentally. The exact figure comes from dividing ln(2) by ln(1 + rate).
Between 6% and 10% the difference is tiny, but at very low rates (under 2%) or very high rates (over 20%) the gap can widen. This calculator shows both values together so you can see the difference immediately.
If the return is 0% or below, the principal only shrinks and never doubles, so no calculation is possible. Enter a rate greater than 0%.