Rule of 70 Calculator

About this calculator

The rule of 70 is a mental-math shortcut for estimating doubling time: divide 70 by the annual growth rate (as a percentage) to get roughly how many years it takes a quantity to double. It comes from the natural-log identity ln(2) ≈ 0.693, so the exact doubling time is ln(2)/r; multiplying by 100 to convert r from a decimal to a percentage gives 69.3/r, which rounds to 70 because 70 divides evenly by more small numbers (2, 5, 7, 10) than 69.3 does, making the mental arithmetic easier.

Economists use it to estimate how long it takes GDP or a population to double at a given growth rate, and to gauge how fast inflation erodes purchasing power (the “doubling” of prices, or halving of what money buys); biologists use it for bacterial or cell-culture growth; and investors use it as a quick sanity check on compound-interest projections alongside the closely related rule of 72. Enter a growth rate to see the estimated doubling time.