Rule of 72

Doubling-time estimate for compound growth, with a reminder that it is an approximation best near mid-single-digit rates.

Name	Formula	Note
Years to double	`years ≈ 72 / r` r is the annual percent rate (use 8 for 8%, not 0.08).	r is the annual percent rate (use 8 for 8%, not 0.08).
Rate for a horizon	`r ≈ 72 / years` What constant percent would double in that many years.	What constant percent would double in that many years.
Exact doubling	`years = ln(2) / ln(1 + r/100)` Closed form when the rate is constant.	Closed form when the rate is constant.
Accuracy note	`best near r ≈ 6% to 10%` Far from that band, use the exact logarithm form.	Far from that band, use the exact logarithm form.

Open a calculator and put these formulas to work.