Rule of 72 Calculator
Enter an expected annual return, and quickly see how many years it takes for your money to double.
How to use the calculator
- Enter the expected annual return as a percentage (e.g. average stock market return or interest rate).
- The calculator instantly shows the mental-math estimate from the Rule of 72, plus the mathematically exact answer for comparison.
What is the Rule of 72?
Worked example: at a 6 % annual return, money doubles in roughly 72 ÷ 6 = 12 years. The exact answer is ln(2) ÷ ln(1.06) = 11.90 years, so the rule of thumb is off by about a month. It is most accurate between 6 and 10 %; at very high rates it becomes noticeably imprecise.
The Rule of 72 is a classic rule of thumb for quickly estimating how long it takes for an investment to double at a given fixed annual interest rate/return, without having to work out the exact logarithmic formula in your head. It's surprisingly accurate for rates between roughly 6% and 10%, and gives a good estimate even outside that range.
The exact formula is ln(2) ÷ ln(1 + rate/100), which gives a slightly more precise answer — the calculator shows both so you can see how close the Rule of 72 actually gets.
Frequently asked questions
How accurate is the Rule of 72?
It's most precise for return rates between about 6% and 10% per year, with a deviation of under a few weeks from the exact answer. Outside that range (very low or very high rates), the deviation increases somewhat, but it still gives a usable mental-math estimate.
Why the number 72 specifically?
72 was chosen because it has many small factors (2, 3, 4, 6, 8, 9, 12, etc.), making it easy to divide mentally for typical interest rates — and because it's close to the mathematically "correct" constant (ln(2) × 100 ≈ 69.3) within the practical interest-rate range.
Does the rule work for inflation too?
Yes, same principle — the Rule of 72 can be used to estimate how many years it takes for the price level (or purchasing power that's halved) to double at a given annual inflation rate.