Rule of 72 calculator tells you how much time it takes for something to double given a certain level of constant growth rate. The term "doubling time" can be used interchangeably - our tool could as well be called "doubling time calculator", because that's exactly what it does. There's a nifty hack that lets you easily do it manually, see below to find out what is rule of 72. This is a special kind of compound interest where you want to work out how long it will take for something (for example your investment) to increase by 100%. It's a fairly narrow case, but it comes up quite often.

What is the rule of 72?

The rule states that in order to get the estimated doubling time, simply divide 72 by a percentage growth in a single period. For example, if something grows by 5% per minute, then it will double in roughly 72 / 5 = 14.4 minutes. The actual number is closer to 14.2067 - this should give you an idea of how much off you may be when you use the rule of a thumb method.

Things to consider when using the doubling time calculator

  • It gives you exact number (well, it's usually rounded), while "manual" rule of 72 gives you a ballpark figure.
  • Rule of 72 calculator is unit-agnostic. It doesn't matter if we're talking about seconds, minutes, days or dog years.
  • It works just as well in reverse. For example, if you want to double your web traffic in 52 weeks, you know it needs to grow by 1.3419% per week.
  • It assumes that the percentage growth is exactly the same each period. So it works awesome with simple interest (when it basically boils down to compound annual growth rate), but with fluctuating percentages (such as your web traffic's increase) it's never stable, so known the limitations of this calculation.
Mateusz Mucha

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Rule of 72 Calculator