The Rule of 72: How to Double Your Money Without the Guesswork

When I talk about investing, people often ask, “How long will it take my money to double?”

It may sound like a complicated math problem, but there is a simple rule of thumb that gives you a quick estimate: The Rule of 72.

What Is the Rule of 72?The Rule of 72 is a shortcut formula that helps you figure out how long it will take your money to double at a given rate of return.

Here’s how it works:

• Take the number 72.

• Divide it by your annual rate of return.

• The result is roughly how many years it will take for your money to double.

Quick Examples

• If your investments earn 6% per year: 72 ÷ 6 = 12 years to double.

• If your investments earn 8% per year: 72 ÷ 8 = 9 years to double.

• If your investments earn 4% per year: 72 ÷ 4 = 18 years to double.

Why It Matters

Understanding the Rule of 72 helps you:

• See the power of compound growth

• Compare different investment options quickly

• Recognize how higher returns can speed up your progress — and how lower returns slow it down

It’s not a crystal ball (markets always fluctuate), but it’s a useful way to visualize the long-term impact of investing.

Try This Today

• Take one of your investment accounts and note the average return.

• Apply the Rule of 72 and calculate how long it might take to double.

• Ask yourself: does that timeline match your goals?

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Katie’s Key Takeaway

The Rule of 72 is a reminder of why starting early matters. The sooner you invest, the more time your money has to double again and again.