The Rule of 72: How to Estimate When Your Money Doubles
The Rule of 72 is one of the most useful mental shortcuts in personal finance. It tells you approximately how many years it takes for an investment to double in value, given a fixed annual return.
How It Works
Divide 72 by the annual interest rate to get the approximate doubling time.
Years to double = 72 ÷ Annual Return Rate
At 8% return: 72 ÷ 8 = 9 years
Examples
| Annual Return | Years to Double |
|---|---|
| 4% | 18 years |
| 6% | 12 years |
| 8% | 9 years |
| 10% | 7.2 years |
| 12% | 6 years |
| 24% (credit card) | 3 years (debt doubles!) |
The Rule of 72 Works in Reverse for Debt
The same rule applies to debt. A credit card at 24% APR will double the amount you owe in just 3 years if you're only paying minimums. This is why high-interest debt is so dangerous.
Why This Matters for Long-Term Investing
$10,000 invested at 8% doubles to $20,000 in 9 years, then $40,000 in 18 years, then $80,000 in 27 years. Starting early is everything — each doubling is exponentially more powerful.
Calculate Exact Compound Growth
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