Enter a rate or a target number of years to estimate how long money takes to double, with two related quick-estimate variants shown alongside.
How to Use the Rule of 72 Calculator
Switch the toggle between "Find Years to Double" and "Find Required Rate." In the first mode, enter an annual interest rate; in the second, enter a target number of years. Alongside the main result, the Rule of 70 and Rule of 69.3 variants show for comparison.
The Rule of 72 is a mental-math shortcut, not a precise formula — division is fast, exponents aren't, and 72 happens to divide cleanly by a lot of common interest rates (6, 8, 9, 12), which is most of why it stuck.
Years to Double = 72 ÷ Rate
Required Rate = 72 ÷ YearsAn 8% rate implies roughly 9 years to double — 72 ÷ 8. Flip it around and a 9-year doubling target implies a required rate near 8%, the same relationship worked from the other direction.
Why 70 and 69.3 Show Up Too
72 approximates a more exact underlying constant, ln(2) × 100 ≈ 69.3, which is exact only under continuous compounding. The Rule of 70 splits the difference and is common in economics texts for population and inflation growth. The Rule of 69.3 is the mathematically precise version for continuous compounding; 72 trades a small amount of accuracy for an easier mental division. At realistic annual rates the three numbers land within a few tenths of a year of each other — close enough that the shortcut is genuinely usable, not just a party trick.
When the Shortcut Breaks Down
Accuracy degrades at very high or very low rates — the approximation is tightest somewhere in the middle single digits and drifts the further out it goes. For anything beyond a rough gut check, especially at double-digit rates, use an actual compound interest calculation instead.