What Is the Rule of 70? Definition, Example and Calculation

What Is the Rule of 70?

The rule of 70, also known as doubling time, calculates the years it takes for an investment to double in value. The calculation is commonly used to compare investments with different annual interest rates.

Key Takeaways

  • The Rule of 70 is a calculation that determines how many years it takes for an investment to double in value based on a constant rate of return.
  • Investors use this metric to evaluate various investments, including mutual fund returns and the growth rate for a retirement portfolio.
  • The Rule of 70 is an estimate that assumes a constant growth rate that may fluctuate, and the calculation may prove inaccurate.

Rule of 70

How to Calculate the Rule of 70

  • Obtain the annual rate of return or growth rate on the investment or variable.
  • Divide 70 by the annual rate of growth or yield.

# of Years to Double an Investment = 70/Annual Rate of Return

Examples of the Rule of 70

  • At a 3% growth rate, a portfolio will double in 23.33 years because 70/3=23.33 
  • At an 8% growth rate, a portfolio will double in 8.75 years because 70/8=8.75 
  • At a 12% growth rate, a portfolio will double in 5.8 years because 70/12=5.8 

What Does the Rule of 70 Tell You?

The Rule of 70 helps investors determine the future value of an investment. Although considered a rough estimate, the rule provides the years it takes for an investment to double. The Rule of 70 is an accepted way to manage exponential growth concepts without complex mathematical procedures.

Investors can use this metric to compare investments with different growth rates or annual returns. If the calculation yields a result of 15 years, an investor looking to double his money in 10 years could make allocation changes to their portfolio to attempt to increase the rate of return.

Rule of 70 vs. Real Growth

The rule evaluates investments but can also estimate other economic factors such as population growth or gross domestic product (GDP). The Rule of 70 is an estimate based on a forecasted growth rate. If future rates fluctuate, the original calculation will be inaccurate.

As of March 2023, the population of the United States was approximately 336 million. A 2020 prediction estimates that the U.S. population will grow at a rate of .62% annually. Using the estimation of the Rule of 70, the population of the U.S. will double in 113 years.

Real growth figures dispute the use of the Rule of 70 in estimating population growth. In 1955, the population of the United States was approximately 172 million and is estimated to double by 2025 based on actual population counts and rates of growth. If the Rule of 70 was used in 1955 to predict the doubling of the population when the growth rate was 1.57%, the population would have doubled by 1999. 

Compound Interest and the Rule of 70

Compound interest is calculated on the initial principal and the accumulated interest of previous periods. The rate at which compound interest accrues depends on the frequency of compounding. The higher the number of compounding periods, the greater the compound interest.

Compound interest is a feature in calculating the long-term growth rates of investments and the various rules of doubling. If the interest earned is not reinvested, the number of years it'll take for the investment to double will be higher than a portfolio that reinvests the interest earned.

The Rule of 70 and any other doubling rules include estimates of growth rates or investment rates of return. As a result, the rule can generate inaccurate results with its limited ability to forecast future growth.

What Is a Limitation of the Rule of 70?

The Rule of 70 assumes a constant rate of growth or return. As a result, the rule can generate inaccurate results since it does not consider changes in future growth rates.

How Is the Rule of 70 Used In Economics?

The Rule of 70 can estimate how long it would take a country's gross domestic product (GDP) to double. Instead of estimating compound interest rates, the GDP growth rate is the divisor of the rule. For example, if the growth rate for China is estimated as 10%, the Rule of 70 predicts it would take seven years, or 70/10, for China's real GDP to double.

What Is the Difference Between the Rule of 70 and the Rules of 69 or 72?

The Rule of 72 or the Rule of 69 may also be used. The function is the same as the rule of 70 but uses 72 or 69, respectively, in place of 70 in the calculations. The Rule of 69 is often considered more accurate when addressing continuous compounding processes, and 72 may be more accurate for less frequent compounding intervals.

The Bottom Line

The Rule of 70 is a calculation that provides an estimate, based on a constant growth rate, of how many years it takes for an investment to double in value. Investors may use this calculation to evaluate the investment returns of mutual funds and retirement portfolios.

Article Sources
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  1. Worldometers. "United States Population."

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