What is the Rule of 72 in Investing?
When evaluating a potential investment—whether it's an index fund, a high-yield savings account, or a real estate syndication—the most common question investors ask is: "How long will it take me to double my money?"
Calculating compound interest usually requires complex logarithmic formulas or a dedicated financial calculator. However, there is an incredibly accurate mental math shortcut that Wall Street professionals have used for decades.
It is called the Rule of 72.
How the Rule of 72 Works
The Rule of 72 allows you to quickly estimate the number of years it will take to double an investment at a fixed annual rate of return.
The Formula: 72 ÷ Annual Interest Rate = Years to Double
Note: You do not convert the interest rate to a decimal. If the rate is 8%, you divide 72 by the number 8.
Real-World Examples
Let's say you inherit $10,000 and you want to know how long it will take to turn that into $20,000. It depends entirely on where you park the cash.
- The Stock Market: Historically, an S&P 500 index fund returns about 10% annually.
- 72 ÷ 10 = 7.2 Years.
- High-Yield Savings Account: Let's assume you find a great savings account paying 4.5% interest.
- 72 ÷ 4.5 = 16 Years.
- Traditional Bank Account: If you leave it in a massive commercial bank paying a dismal 0.1% interest.
- 72 ÷ 0.1 = 720 Years.
Why It Is Useful
The Rule of 72 is powerful because it highlights the staggering cost of leaving your money in low-interest accounts.
If you leave your money under your mattress, inflation will cut its purchasing power in half over a decade. If you invest it at an 8% to 10% return, you are doubling your wealth every 7 years without lifting a finger.
See the Full Picture
While the Rule of 72 is a fantastic party trick for estimating doubling times, it doesn't account for ongoing monthly contributions (like putting an extra $200 a month into your account).
To see a complete, year-by-year breakdown of exactly how your portfolio will grow based on your monthly deposits, run your numbers through our free Investment Calculator!