A quick mental math shortcut for estimating how long it takes money to double — no calculator needed.
Divide 72 by your annual interest rate to get the approximate number of years it takes to double your money.
| Rate | Years to double | Actual (exact) |
|---|---|---|
| 2% (HYSA, low yield) | 36 years | 35.0 years |
| 4% (CDs, bonds) | 18 years | 17.7 years |
| 7% (stocks, inflation-adjusted) | 10.3 years | 10.2 years |
| 10% (stocks, nominal) | 7.2 years | 7.3 years |
| 22% (credit card rate) | 3.3 years | 3.5 years |
The exact formula for doubling time is ln(2) ÷ ln(1+r). For rates between 2-15%, 72 divided by the rate gives an excellent approximation because ln(2) ≈ 0.693 and ln(1+r) ≈ r for small r — making the ratio ≈ 0.693/r. Multiplying by 100 gives 69.3/rate%, and 72 is a rounder number that's slightly more accurate in the 6-10% range most investors care about.
The calculator shows exactly how your money compounds over time.