Rule of 72 — Doubling Time Calculator
At 8% your money doubles in about 9 years — 72 ÷ 8. Check the shortcut against the exact formula, in either direction.
Doubling times at common returns
| Annual return | Rule of 72 | Exact | Typical of… |
|---|---|---|---|
| 2% | 36 yrs | 35.0 yrs | Inflation target / savings account |
| 4% | 18 yrs | 17.7 yrs | Bonds, conservative portfolio |
| 7% | 10.3 yrs | 10.2 yrs | Stock market (inflation-adjusted, long run) |
| 10% | 7.2 yrs | 7.3 yrs | Stock market (nominal, long run) |
| 20% | 3.6 yrs | 3.8 yrs | Exceptional — sustained by almost no one |
Frequently asked questions
Why 72?
The exact math says ln(2) ÷ ln(1+r) ≈ 69.3 ÷ r for small rates, but 72 divides cleanly by 2, 3, 4, 6, 8, 9 and 12 — and the small overshoot happens to correct for compounding at everyday rates. Between 4% and 12% the rule lands within about 2% of the true answer.
Does it work for inflation and debt too?
Yes — anything compounding. At 6% inflation, prices double (money halves) in ~12 years; a debt at 24% APR doubles in ~3 — the same arithmetic that grows investments works against you on credit cards.
What about quadrupling?
Doubling twice: just double the doubling time. At 8%, money doubles in ~9 years and quadruples in ~18 — the quiet magic of compounding is that the second double takes no longer than the first.
Last updated: 2026-07-13