The Rule of 72 is a fast mental shortcut for how long it takes money to double at a given compound rate: divide 72 by the annual rate and you get the number of years. Enter your expected annual interest or return rate and this calculator gives you the doubling time instantly, and it also works the reverse way, showing what rate you would need to double your money in a target number of years. It is an approximation, most accurate for rates in the middle single digits, but it is remarkably close and needs no complex math, which is why investors use it to sanity-check returns on the fly. Try different rates to feel how much faster a few extra percent compounds. It all runs in your browser.