Rule of 70 Calculator
Economists' Preferred Version of the Doubling-Time Shortcut
The Rule of 70 serves the same purpose as the more familiar Rule of 72, but economists tend to reach for it specifically when discussing growth rates like inflation, GDP, or population — contexts where the underlying rate is usually a smaller, more continuous figure. Dividing 70 by the annual growth rate approximates how many years it takes a quantity to double, and it derives more directly from the natural-log relationship behind continuous compounding.
The Formula
The constant 70 approximates 100 × ln(2), which is about 69.3 — rounded to 70 for easy mental division by common growth rates like 1%, 2%, 5%, and 7%, similar to how 72 is favored for interest-rate math because of its divisibility.
Where This Matters
- Inflation projections — a 3.5% average inflation rate implies prices double roughly every 20 years, a figure that puts long-term retirement planning in concrete terms.
- Population and GDP growth — demographers and economists commonly cite doubling times derived from this rule when discussing national or global growth trends.
- Comparing to the Rule of 72 — for rates below about 5%, the Rule of 70 tends to track the true doubling time slightly more closely than the Rule of 72.
| Annual growth rate | Years to double (Rule of 70) |
|---|---|
| 1% | 70.0 |
| 2% | 35.0 |
| 3.5% | 20.0 |
| 5% | 14.0 |
| 7% | 10.0 |
| 10% | 7.0 |
How to Use This Calculator
- Enter the annual growth rate as a percentage.
- Select Calculate to see the estimated number of years for the quantity to double.
Related Calculations
Compare against the Rule of 72 Calculator, the more common version for investment returns, or use the Present Value Calculator to work with exact figures instead of an approximation.