Using the Rule of 72 to Estimate Investment Returns

The Rule of 72 is a Quick Calculation to See How Fast Your Money Doubles

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Compound interest is an amazing thing, and the Rule of 72 is a simple way to quickly estimate how long it will take your investment to double in size, assuming you reinvest any dividends.

The only piece of information you need for this calculation is the annual rate of return, although to do the math, you'll probably also want a calculator.

While most investments don’t have a fixed rate of return over a long period of time, you can use an average estimate to get a pretty good idea of how long it will take you to double your money.

How to Use the Rule of 72

To estimate how long it takes for your money to double, simply divide 72 by the interest rate. The result is how many years it will take for your money to double at that rate.

For example, let’s assume you can earn a 6% rate of return. How long will it take $1,000 to grow into $2,000? Here's the equation:

72 / 6 = 12 years

In this example, if you invested $1,000 into an account that earned a flat 6% annual rate of return, after 12 years, your investment would be worth around $2,000.

The Rule of 72 by Interest Rate

To save you a little time, here are some common interest rates, plus the amount of time it will take for you to double your investment with each interest rate:

1% - 72 years
2% - 36 years
3% - 24 years
4% - 18 years
5% - 14 years
6% - 12 years
7% - 10.3 years
8% - 9.0 years
9% - 8.0 years
10% - 7.2 years
11% - 6.5 years
12% - 6.0 years

Remember, It’s Just an Estimate

Keep in mind that this is just a quick estimate.

Depending on changes in the rate of return over time, what you’re invested in, how you invest it, how interest is applied, and possible tax implications, the actual amount of time needed to double your money will vary.

Even so, the rule of 72 can be helpful when you quickly want to compare the rate of growth of two investments.

The rule of 72 also works in reverse — it can be helpful in understanding the power of inflation. If you consider the average long-term rate of inflation is between 3% and 4%, you’ll notice when you use the rule of 72 that something worth $100 today will cost $200 in about 20 years.

This can help illustrate the power of inflation, and the importance of realizing a rate of return over time that can not only overcome inflation, but also taxes.