When does money double, when does purchasing power halve?

There is a rule of thumb for doing this in your head: 72 divided by the rate gives the years until an amount doubles.

It works in both directions. At an interest rate it tells you when a balance has doubled, at an inflation rate when the price level has doubled, and that is exactly the point where your money is worth half.

The rule is an approximation. Next to it stands the exact figure, which is worked out with a logarithm instead of a division.