Doubling Time Calculator

About this calculator

Doubling time is the length of time it takes a quantity growing at a constant exponential rate to double in size, calculated with the formula t = ln(2) ÷ r, where r is the growth rate per period expressed as a decimal and ln(2) is approximately 0.693. It applies to anything that compounds at a steady percentage rate — investment returns, population growth, inflation, bacterial cultures, or a business’s recurring revenue.

This exact formula is the source of the popular Rule of 70 (and its close cousin the Rule of 72) approximation, which simply multiplies 0.693 by 100 and rounds to a convenient whole number for quick mental-math estimates. The doubling time calculation gives the precise figure rather than the rough approximation, useful whenever growth rates are unusual — very high, very low, or needing to be compared precisely across different scenarios.