Rule of 72 Calculator
Divide 72 by the rate — and see how close the shortcut is to the exact answer.
Quick answer
Rule of 72: years to double = 72 ÷ annual interest rate. At 6% money doubles in about 12 years (exactly 11.9), at 8% in about 9 years (exactly 9), and at 10% in about 7.2 years (exactly 7.3). It also works backwards: to double in 6 years you need about 12% a year.
Years to double (rule of 72)
9
- Exact years to double
- 9.01
- Rule of 70 estimate
- 8.8
- Years to triple (rule of 114)
- 14.3
- Years to quadruple (rule of 144)
- 18
Assumes the rate compounds once a year and stays the same.
Years to double: rule of 72 vs exact
| Rate | Rule of 72 | Exact | Difference |
|---|---|---|---|
| 2% | 36 | 35 | 1 yr |
| 3% | 24 | 23.45 | 0.55 yr |
| 4% | 18 | 17.67 | 0.33 yr |
| 5% | 14.4 | 14.21 | 0.19 yr |
| 6% | 12 | 11.9 | 0.1 yr |
| 7% | 10.3 | 10.24 | 0.04 yr |
| 8% | 9 | 9.01 | 0.01 yr |
| 10% | 7.2 | 7.27 | 0.07 yr |
| 12% | 6 | 6.12 | 0.12 yr |
| 15% | 4.8 | 4.96 | 0.16 yr |
| 20% | 3.6 | 3.8 | 0.2 yr |
How it works
The rule of 72 is a mental shortcut for compound growth. Divide 72 by the yearly rate and you get roughly how many years it takes for money to double. The exact answer is ln(2) ÷ ln(1 + rate), and between about 4% and 12% the shortcut is within a few months of it.
The same idea runs in reverse for anything that grows against you. At 3% inflation, prices double in about 24 years — so money held as cash loses half its buying power over that time. Credit card debt at 24% doubles in about 3 years if nothing is paid.
Variants exist: the rule of 70 is slightly more accurate at low rates, and the rule of 69.3 is exact for continuous compounding. To triple money, use 114; to quadruple, 144. For a full year-by-year picture with regular deposits, use the compound interest calculator. To plan monthly saving toward a specific amount, use the sinking fund calculator.
More saving & investing calculators
Frequently asked questions
How accurate is the rule of 72?
Very close for rates between about 4% and 12% — usually within a few months. It overestimates slightly at low rates and underestimates at high rates.
Why 72 and not 70?
72 divides evenly by many common rates (2, 3, 4, 6, 8, 9, 12) and is more accurate around 8%. The rule of 70 is a little better for low rates such as inflation.
How long does it take to double money at 7%?
About 10.3 years by the rule of 72, and 10.24 years exactly.