Most people struggle to understand the concept of compounding because our brains are wired for linear growth. If you earn $100 a year, you’ll have $1,000 in ten years. Simple.

But compounding is exponential. Your interest earns interest. Eventually, the growth of the growth starts to outpace your original contributions. The "Rule of 72" is a mental math shortcut to help you visualize exactly how powerful time can be.

The Formula

72 ÷ R = Y

72 divided by your Rate of Return (R) equals the number of Years (Y) to double your money.

For example, if you invest $1,000 in an index fund that average 7% returns per year:

72 ÷ 7 = ~10.3 Years

In roughly 10 years, your $1,000 becomes $2,000 without you lifting a finger.

Reverse Engineering: Debt

The scary part? The Rule of 72 works for your creditors, too. If you have a credit card balance at 24% interest and you don't make any payments:

72 ÷ 24 = 3 Years

In just three short years, your debt doubles. This is why high-interest debt feels so inescapable—the math is working against you at triple speed.

"Compound interest is the eighth wonder of the world. He who understands it, earns it; he who doesn't, pays it." — (Attributed to) Albert Einstein

Why it matters for students

As a student, you have the one asset that billionaires can't buy more of: Time.

Because of the Rule of 72, a single $1,000 investment made at age 20 could double five or six times before you retire. By age 30, that same $1,000 would only have time to double four times. That ten-year delay could cost you hundreds of thousands of dollars in the long run.