Saving and investing can be full of unknowns. How fast will your money grow? When you use an account with a fixed interest rate or an investment with a fixed rate of return, use the Rule of 72 to calculate how long it will take for your money to double. 

What is the Rule of 72?

The Rule of 72 is a simple formula that tells you how long it will take to double your investment. 

The calculation is easy: just divide the annual interest rate by 72. The answer will be the number of years it will take to double your funds.

The Rule of 72 at work

Here’s an example: Let’s say you invest $1,000 in a 20-year US Treasury bond that earns a 2.18% fixed annual interest rate. Here’s the math for the Rule of 72: 

72 divided by 2.18 = 33.6 years. 

In a 20-year Treasury bond, it would take about 33 and a half years to double your money. 

For the record, if that feels like a long time, Treasury bonds are designed as secure and stable investments. They’re protected by the US government, so they’re a safe place to keep money, and earning a fixed rate provides secure growth too. There’s also no limit on their government protection, so you can safely store more than a CD or savings account, which carry a $250,000 FDIC limit. 

The Rule of 72 only works with fixed or very low interest rates, and when the rate of rate varies, like on stocks or some mutual funds, the formula doesn’t work at all. 

Some investments can increase in value quickly, just like they can drop in value too. For those, projecting growth requires factors and formulas more complex than the Rule of 72.

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Bright is a good place to start before investing or when you’re trying to build your savings fast. Bright builds a personal financial plan tailored to every user, based on your finances, debts and goals. 

Powered by MoneyScience™, Bright studies your income and expenses, learns about your goals and makes smart credit card payments for you, while growing your savings week by week, automatically. Bright does it all for you and even adjusts when your finances shift. 

If you don't have it yet, download the Bright app from the App Store or Google Play. Connect your checking account and your credit cards, set a few goals and let Bright get to work!

Recommended Readings:

When is a savings account the best investment?

How to choose the right investment for you