Rule of 72 Calculator
The Rule of 72 is the famous mental-math shortcut for compounding: divide 72 by your annual return and you get, roughly, the number of years it takes to double your money. It’s a fast, surprisingly accurate way to grasp the power of compound growth.
How this calculator works
The rule is just a division:
years to double ≈ 72 ÷ annual return (%)
At 8% a year, money doubles in about 72 ÷ 8 = 9 years. You can also flip it to find the rate needed to double by a target date: rate ≈ 72 ÷ years.
It’s an approximation of the exact compound formula ln(2) ÷ ln(1 + r), which this calculator also shows. The rule is most accurate for returns in the mid-single-digits to low-teens — right where most long-term investment assumptions sit.
Why it’s useful
The Rule of 72 turns abstract percentages into something intuitive. It instantly shows why a few extra percentage points of return — or a few percent of inflation working against you — matters enormously over a lifetime. For the full picture of growth with contributions, use the compound interest and retirement calculators.
Frequently asked questions
- How accurate is the Rule of 72?
- Very, for typical rates. Between about 6% and 10% it's within a fraction of a year of the exact answer. At very high or very low rates it drifts a bit — this calculator shows the exact figure alongside for comparison.
- Can I use it for inflation too?
- Yes. Divide 72 by the inflation rate to estimate how many years until prices double (and your cash's buying power halves). It works the same in reverse for anything that compounds.
- Why 72 and not another number?
- 72 is a convenient choice because it divides cleanly by many common rates (2, 3, 4, 6, 8, 9, 12) and closely matches the math. Some use 69 or 70 for continuous compounding; 72 is the practical favorite.