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Rule of 70 Doubling Time Calculator

Enter a growth rate per period to estimate the doubling time.

How to use

  1. Enter your values in the fields above.
  2. Press Calculate to see your result instantly.
  3. Use the Share button to copy a link to your result.

About this calculator

The Rule of 70 is a quick mental-math shortcut for estimating how long it takes a quantity growing at a steady percentage rate to double in size: just divide 70 by the growth rate. It comes from the exact doubling-time formula ln(2)/r ≈ 0.693/r, and 70 is used because it divides evenly by more common growth rates than the mathematically more precise 69.3.

This calculator estimates doubling time from any growth rate and shows how the Rule of 70 compares against the closely related Rule of 72 (often preferred in finance because 72 has more divisors, like 6, 8, and 9) and the exact 69.3 figure. It's widely used in economics and investing to gauge how quickly compound interest will double an investment, and in demography to estimate how fast a growing population will double.

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