Based on an annual increase of 6%, my water bill will double every 12 years. This is the sick reality of living in Toronto and environs.
In finance, the rule of 72, the rule of 70 and the rule of 69.3 are methods for estimating an investment’s doubling time. The rule number (e.g., 72) is divided by the interest percentage per period to obtain the approximate number of periods (usually years) required for doubling. Although scientific calculators and spreadsheet programs have functions to find the accurate doubling time, the rules are useful for mental calculations and when only a basic calculator is available.