The Rule of 72: How to Estimate When Your Money Will Double

The Rule of 72 is arguably the most useful mental math shortcut in all of finance. It gives you a quick, surprisingly accurate estimate of how long it takes an investment to double in value — no calculator needed.

The formula: Years to double ≈ 72 ÷ Annual rate of return. That's it. Investing at 6%? Your money doubles in roughly 12 years (72 ÷ 6 = 12). Earning 10%? About 7.2 years.

Rule of 72 Quick Reference Table

Here is how the Rule of 72 maps across common rates:

• 2% (savings account): 36 years to double
• 4% (bonds): 18 years to double
• 6% (balanced portfolio): 12 years to double
• 7% (S&P 500 real return): 10.3 years to double
• 8% (aggressive growth): 9 years to double
• 10% (S&P 500 nominal return): 7.2 years to double
• 12% (high-growth stocks): 6 years to double

Using the Rule of 72 in Reverse

The rule works both ways. If you want to double your money in a specific timeframe, divide 72 by the number of years to find the required rate of return.

Want to double a college fund in 10 years? You need 72 ÷ 10 = 7.2% annual return. Want to double in 5 years? You need 72 ÷ 5 = 14.4% — which tells you that you either need aggressive investments or more realistic expectations.

The Rule Applied to Inflation and Debt

The Rule of 72 is not just for investments. It tells you how fast inflation erodes your purchasing power: at 3.5% inflation (roughly the 2026 rate), your dollar's buying power halves in about 20.6 years. Money sitting in a 0.5% savings account effectively loses half its value in a generation.

It also reveals the true cost of high-interest debt. Credit card debt at 24% APR doubles in just 3 years (72 ÷ 24 = 3). A $5,000 balance you ignore becomes $10,000 in three years, $20,000 in six. This is why paying off high-interest debt is mathematically equivalent to earning that rate of return risk-free.

Where the Rule Breaks Down

The Rule of 72 is an approximation. It is most accurate for rates between 4% and 12%. At very low rates (below 2%) or very high rates (above 20%), the estimate drifts. For more precision at low rates, use the Rule of 69.3 (divide 69.3 by the rate instead). For rates above 20%, use the Rule of 78.

The rule also assumes a constant rate of return, which never happens with real investments. Market returns are lumpy — you might earn 25% one year and lose 10% the next. Over long periods, the average tends to work out, but in any given decade, your actual doubling time can vary significantly from the estimate.

Real-World Application: How $10,000 Grows

Let's trace $10,000 invested at age 25 with a 7% average annual return (approximately the S&P 500's historical real return after inflation):

• Age 35 (10 years): ~$20,000 (1st doubling)
• Age 45 (20 years): ~$40,000 (2nd doubling)
• Age 55 (30 years): ~$80,000 (3rd doubling)
• Age 65 (40 years): ~$160,000 (4th doubling)