Calculate how long it takes for an investment, population, or any growing quantity to double in size. Uses both the Rule of 70 approximation and exact logarithmic formula.
Enter the annual growth rate (e.g., 7 for 7% per year)
Problem: An investment grows at 7% per year. How long until it doubles?
Rule of 70: 70 ÷ 7 = 10 years
Exact: ln(2) ÷ ln(1.07) = 9.9 years
Calculate how long it takes for investments to double with compound interest.
Estimate when populations will double based on current growth rates.
Project when revenue, customers, or market size will double.
Analyze GDP growth, inflation effects, and economic doubling periods.
| Growth Rate | Rule of 70 | Exact Time | Error |
|---|---|---|---|
| 1% | 70.00 | 69.66 | 0.5% |
| 2% | 35.00 | 35.00 | 0.0% |
| 3% | 23.33 | 23.45 | 0.5% |
| 5% | 14.00 | 14.21 | 1.5% |
| 7% | 10.00 | 10.24 | 2.3% |
| 10% | 7.00 | 7.27 | 3.7% |
| 15% | 4.67 | 4.96 | 5.8% |
| 20% | 3.50 | 3.80 | 7.9% |
The Rule of 70 is a quick way to estimate doubling time by dividing 70 by the growth rate percentage. It's quite accurate for growth rates between 1% and 10%, with less than 1% error for most rates in this range.