Why 72?
The exact doubling time comes from logarithms: ln(2) ÷ ln(1 + rate). That's not something you can do in your head. The number 72 is a clever approximation — it divides cleanly by 2, 3, 4, 6, 8, 9 and 12, and it's most accurate for rates in the 6–10% range that matter most to investors. At 8%, the rule says 9 years; the exact answer is 9.01. Remarkably close for mental math.
Everyday uses
- Investing: at an 8% average return, money doubles roughly every 9 years — so a 27-year horizon means about three doublings, an 8x increase.
- Inflation, in reverse: at 3% inflation, prices double (your money's purchasing power halves) in about 24 years. At 6%, just 12.
- Debt, as a warning: a 24% credit card balance left unpaid doubles what you owe in about 3 years.
Accuracy and limits
For higher rates the rule drifts: at 20%, it estimates 3.6 years vs an exact 3.8. Some people switch to 70 for continuous compounding or 69.3 for maximum precision, but 72 wins on mental-math convenience. The bigger caveat: real investments don't return a steady rate — they average one out through violent ups and downs. The Rule of 72 tells you what a smooth average implies, which is a planning guide, not a prediction of any single year.