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What is the Rule of 70?
The Rule of 70 is a simple formula used to estimate how long it will take for a value (such as GDP, investment, or population) to double, given a constant annual growth rate.
Key takeaway: Divide 70 by the annual growth rate to find the approximate number of years it will take for something to double in size or value.
Definition
The Rule of 70 estimates the doubling time of an investment or economic variable by dividing 70 by its annual percentage growth rate.
Why It Matters
The Rule of 70 helps investors, economists, and policymakers quickly understand the effects of compounding growth. It simplifies long-term forecasting and illustrates how small differences in growth rates can lead to large changes over time.
Key Features
- Quick and easy mental calculation.
- Applies to investments, GDP, or population growth.
- Based on the principle of exponential growth.
- Works best with steady annual growth rates below 15%.
- Useful for comparing different economic scenarios.
How It Works
- Identify Growth Rate: Determine the annual growth rate (in %).
- Apply Formula: Divide 70 by that rate.
- Interpret Result: The answer is the number of years to double.
- Example: 70 ÷ 7% = 10 years → value doubles in 10 years.
- Compare Growth Scenarios: Use for quick estimation across countries, companies, or assets.
Types
- Investment Growth: Estimate how long your investment will double.
- Economic Growth: Evaluate GDP doubling periods.
- Population Growth: Project demographic expansion.
Comparison Table
| Feature or Aspect | Rule of 70 | Rule of 72 |
|---|---|---|
| Divisor | 70 | 72 |
| Accuracy Range | 0–15% | 0–20% |
| Simplicity | High | High |
| Common Use | Economic analysis | Financial planning |
Examples
- Example 1: A country growing GDP at 3.5% will double output in 20 years (70 ÷ 3.5).
- Example 2: An investment compounding at 10% doubles in 7 years (70 ÷ 10).
- Example 3: A population growing at 2% doubles in 35 years (70 ÷ 2).
Benefits and Challenges
Benefits
- Simple and fast to calculate.
- Useful for long-term projections.
- Helps visualize compounding effects.
- Applicable across disciplines.
Challenges
- Assumes constant growth rates.
- Less accurate at high or volatile rates.
- Doesn’t account for inflation or taxes.
Related Concepts
- Exponential Growth: Continuous compounding effect over time.
- Compound Interest: Growth on both principal and accumulated interest.
- Rule of 72: A similar formula with slightly different accuracy.
FAQ
Why use 70 as the divisor?
It’s derived from the natural logarithm of 2 (≈0.693) multiplied by 100, approximating the doubling time formula.
When is the Rule of 70 most accurate?
For annual growth rates between 1% and 15%.
What’s the difference between the Rule of 70 and 72?
Both estimate doubling time, but the Rule of 72 is more common in finance, while 70 suits economic growth.
Can it apply to negative growth?
No — it’s only valid for positive compounding rates.
Sources and Further Reading
- World Bank: Understanding Economic Growth
- IMF: Macroeconomic Indicators and Doubling Time
- Investopedia: https://www.investopedia.com/terms/r/rule-of-70.asp
Quick Reference
- Doubling Time: Years required for a value to double.
- Growth Rate: Annual percentage increase.
- Exponential Growth: Process of compounding over time.
