Rule of 72 Calculator

Here's a party trick that'll make you look smart at dinner: want to know how long it takes for your money to double? Just divide 72 by your interest rate. That's it. No complicated formulas, no spreadsheets, no calculator gymnastics.

Let's say you're earning 8% on an investment. 72 divided by 8 equals 9. Your money will double in roughly 9 years. It's not perfect, but it's close enough to make quick decisions.

How does the Rule of 72 work?

The Rule of 72 is essentially a shortcut for a much messier logarithmic equation that nobody wants to calculate in their head. Someone way smarter than us figured out that 72 has a bunch of factors, which makes the mental math work out nicely for most interest rates people actually encounter (between 6-10%).

Think of it like rounding up when you're splitting a restaurant bill. Sure, you could calculate everyone's exact portion down to the penny, but dividing by 4 when there are actually 3.7 people is close enough and way faster.

Rule of 72 formula

The math couldn't be simpler:

Example: If someone tells you their investment doubled in 12 years, you can quickly figure out they averaged about 6% returns (72 ÷ 12 = 6).

When is the Rule of 72 most accurate?

The Rule of 72 works best for interest rates between 6% and 10%. Go lower or higher, and the estimate starts getting a bit wonky. For super low rates (like 2%), the Rule of 70 is technically more accurate. For rates above 10%, try the Rule of 69. But honestly? The Rule of 72 is good enough for most situations.

Why use the Rule of 72?

Because sometimes you don't need exact answers—you need quick answers. Whether you're comparing investment options at a financial advisor's office or trying to explain compound interest to your teenager, the Rule of 72 gives you an instant ballpark figure.

This Rule of 72 calculator shows you both the quick estimate and the actual doubling time, so you can see how close the shortcut really is. Try plugging in different rates and watch how the Rule of 72 stacks up against the real math.