Rule of 72 Calculator (Investment Doubling Time)
Find
Doubling at 8% a year
9.00 years
by the rule of 72. With annual compounding the exact answer is 9.01 years (72 ÷ 8; exact: ln 2 ÷ ln(1 + 0.08)), so the shortcut is off by 0.01 years.
| Rule of 72 | 9.00 years |
|---|---|
| Rule of 70 | 8.75 years |
| Rule of 69.3 (continuous compounding) | 8.66 years |
At the same rate money triples in 14.3 years and grows tenfold in 29.9. The rule works for debts too: a 24% credit card balance doubles in about 3 years if unpaid.
How accurate is the rule of 72?
| Rate | 72 ÷ rate | Exact years |
|---|---|---|
| 1% | 72.0 | 69.7 |
| 2% | 36.0 | 35.0 |
| 3% | 24.0 | 23.4 |
| 4% | 18.0 | 17.7 |
| 5% | 14.4 | 14.2 |
| 6% | 12.0 | 11.9 |
| 7% | 10.3 | 10.2 |
| 8% | 9.0 | 9.0 |
| 9% | 8.0 | 8.0 |
| 10% | 7.2 | 7.3 |
| 12% | 6.0 | 6.1 |
| 15% | 4.8 | 5.0 |
| 20% | 3.6 | 3.8 |
The rule of 72 is closest around 8%; the rule of 70 suits low rates such as inflation or GDP growth, and 69.3 is exact for continuous compounding.
Estimate how long an investment takes to double using a quick mental-math approximation for return-rate planning.
Formula
Worked Example
At 8% return, the rule estimates doubling near 9 years, useful for fast scenario benchmarking.
Frequently Asked Questions
How accurate is Rule of 72?
It is a close approximation for moderate return rates, but exact compound calculations are better for final planning.
Can it be used for debt growth?
Yes. The same logic can estimate how quickly debt may double at a given interest rate.