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The Rule of 72 and friends

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The Rule of 72 and friends

The Rule of 72 is a shortcut so elegant and useful that it belongs in your mental toolkit forever. Divide 72 by any annual growth rate, and you get the approximate number of years it takes your money to double. At 8%, it's 9 years. At 10%, it's 7.2 years. At 2%, it's 36 years. At 1%, it's 72 years. The beauty is that the math works across the entire range of rates you're likely to encounter.

This rule works because of the mathematics of logarithms, but you don't need to know why to use it. What matters is that you carry it with you to the grocery store, the coffee shop, the investment meeting. When someone says "inflation runs 3%," you immediately know: your purchasing power halves in 24 years. That's a decade-scale impact on your standard of living. When a fund manager claims a 12% return, you know: your money doubles in six years. You don't need a calculator. You don't need anyone's permission. You just divide and you have clarity.

This chapter extends the rule. There's a Rule of 114 for tripling, a Rule of 144 for quadrupling. For lower rates, 69 or 70 works better than 72. And the rule works in reverse: how much must you earn annually to double your money in a specific time?

Intuition, not precision​

The Rule of 72 is approximate. It's designed for mental math, not to replace your spreadsheet. But that's precisely why it's powerful. When an investment manager claims their fund achieves 9% returns, you need a way to understand what that means over decades. The Rule of 72 gives you that tool immediately: 72 divided by 9 is 8 years. Your money doubles every 8 years. Over 32 years, it quadruples four times over. That's 16 times the starting amount.

An investment manager can hide behind decimal places and small print. The Rule of 72 cuts through. It forces clarity. At what rate does this investment double? Divide. Get an answer in seconds. No calculator, no spreadsheet, no permission needed. You can check claims on the fly, at a dinner table or in a meeting.

Speed and slippage​

Armed with the rule, you'll start seeing compounding everywhere: in salary growth, population, inflation, credit card debt. You'll also start seeing where the rule breaks down: at very high or very low rates, the approximation loses accuracy. A 1% growth rate gives you a doubling time of 72 years; in reality, it's 69.7 years. The rule overshoots slightly. A 20% growth rate gives you a doubling time of 3.6 years; the actual answer is 3.8 years. Again, the rule overshoots. We'll explore the limits and show you when to trust it and when to pull out the calculator, and why the rule is so robust across most real-world scenarios.

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