The Rule of 72 Explained

If someone asks you how long it'll take an investment to double at a given rate of return, you don't need a calculator to give a fast, reasonably accurate answer. The Rule of 72 is a mental-math shortcut that's been used by investors for generations, and it takes about five seconds once you know it.

What is the Rule of 72?

Divide 72 by your annual rate of return (as a whole number, not a decimal), and the result is roughly the number of years it takes your money to double.

Years to double ≈ 72 ÷ annual rate of return

At an 8% annual return, that's 72 ÷ 8 = 9 years to double your money. At 6%, it's 72 ÷ 6 = 12 years. At 12%, just 72 ÷ 12 = 6 years.

Why 72?

The real formula for doubling time comes from natural logarithms: years to double = ln(2) ÷ ln(1 + r). For the rates most people actually invest at, that works out close to 0.70 ÷ r, and 72 is used instead of 70 purely because it divides evenly by more small numbers (2, 3, 4, 6, 8, 9, 12), which makes it far easier to do in your head. That convenience costs a small amount of accuracy, which is the tradeoff below.

How accurate is it, really?

The Rule of 72 is most accurate in the 6% to 10% range, which happens to cover most long-term stock market and diversified portfolio return assumptions. It drifts more the further you get from that range in either direction:

Notice the estimate flips from slightly too slow at low rates to slightly too fast at high rates, with 8% as close to a perfect match as it gets. For anything outside roughly 4% to 15%, treat the Rule of 72 as a ballpark, not a forecast.

It works on debt too, and that's the scary part

The Rule of 72 doesn't care whether the number growing is your investment or your credit card balance. A card charging 24% APR on a carried balance doubles what you owe in roughly 72 ÷ 24 = 3 years if you make no payments beyond letting interest compound. It's the same math working against you instead of for you, which is part of why paying down high-interest debt is often the better "investment" before building an investment portfolio. If you're weighing that tradeoff, the debt payoff calculator shows the exact month-by-month numbers rather than an estimate.

When to use the real calculator instead

The Rule of 72 assumes a single lump sum, a constant rate of return, and no additional contributions along the way, which makes it great for a quick gut-check but not for actually planning around. The moment you're adding money monthly, comparing a few different scenarios, or want to account for inflation eating into the "real" doubling time, you want exact numbers instead of an estimate.

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