Mental maths and numerical fluency

Rule of 72

Doubling time at a percentage growth rate, accurate enough to quote from.

tdouble≈72rt_{\text{double}} \approx \frac{72}{r}

Where

rr
The rate as a percentage, not a decimal.

Assumptions

  • An approximation to ln⁡2/ln⁡(1+r)\ln 2 / \ln(1+r), closest around 8%.
  • 72 is used rather than the more accurate 69.3 because it divides cleanly by more numbers.

Where this is taught