The rule that doubles your money (or shrinks it)

In 1494, the friar Luca Pacioli — the father of modern accounting — wrote down a rule bankers were already using in their heads: divide 72 by the annual interest rate and you get the years it takes for a sum to double.

It works surprisingly well. Earning 6% a year? 72 divided by 6: twelve years. 8%? Nine years. No spreadsheet, no logarithms — schoolyard division.

The unsettling part is that the rule also works against you. With 3% inflation, do the maths: in about twenty-four years prices will have doubled, which means your idle savings will be worth half.

Compound interest is always on the clock; the only question is whose side it's on. The rule of 72 tells you, in a five-second calculation, whether time is playing for you or against you.