For an investment with annually compounded interest the time required for it to double can be quickly estimated by using the ‘rule of 72’ (years to double = 72/percent annual interest). A simple derivation for the rule and an evaluation of its accuracy are presented. Results indicate the rule errs by less than l1/2 months for interest rates between 6 and 12 per cent, and by less than 21/2 months for rates between 5 and 21 per cent, and by less than 31/2 months for 4‐3 per cent or any larger rate.