The purpose of this Note is to provide easily remembered rules for finding approximately the time required for money to multiply itself by any whole number up to 12, and by certain larger numbers, at a given rate of interest. The rules hold good for rates of interest up to about 100% per period. For ease of reference they are summarized at the end of the Note.

It may well be asked why anyone should want to memorize such rules when reference to tables of compound interest, or calculation by logarithms, would give the results very easily. One answer is that in a time of increasing wage and salary levels it is important for the actuary to be able to point out at once the practical effect of any proposed assumption as to future rates of increase. Such information is often needed at a meeting, where the overt or surreptitious consultation of a book of tables would not enhance his reputation.