Using the rule of 70, approximately how many years will it take for a population growing at 2 percent per year to double in size?

• A 140 years
• B 70 years
• C 50 years
• D 35 years ✓

Explanation

The rule of 70 states that doubling time equals 70 divided by the annual percentage growth rate. For a 2 percent growth rate, 70 divided by 2 gives 35 years. Choosing 140 years would result from multiplying rather than dividing, and 70 years would apply only if the growth rate were 1 percent. This shortcut is widely used to estimate the impact of growth rates on population size.

Reference: Park's Textbook of Preventive and Social Medicine, 27th ed.

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