The Rule of 72: A Quick Way to Estimate How Fast Money Doubles
September 18, 2026
The Rule of 72 is a quick mental-math shortcut: divide 72 by an annual interest rate (as a whole number) to estimate how many years it takes an investment to double. At 6%, money doubles in about 72/6 = 12 years. It's an approximation, not an exact formula, but it's remarkably accurate for typical rates.
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Years to Double ≈ 72 / Interest Rate. It's a simplified approximation of the actual compound interest doubling formula, which involves natural logarithms — the number 72 is chosen because it divides evenly by many common rates (2, 3, 4, 6, 8, 9, 12), making the mental math easy.
Rule of 72 at Common Rates
| Annual Rate | Years to Double (Rule of 72) | Actual Years |
|---|---|---|
| 3% | 24.0 | 23.4 |
| 6% | 12.0 | 11.9 |
| 8% | 9.0 | 9.0 |
| 12% | 6.0 | 6.1 |
Where It's Most Useful
The Rule of 72 works best for rates roughly between 6% and 10%, where it's accurate to within a few weeks. It's a quick sanity-check tool for comparing investment options or explaining compound growth without pulling out a calculator — for precise planning, use the full compound interest formula instead.
Frequently Asked Questions
How accurate is the Rule of 72?
Very accurate for rates between about 6% and 10% — typically within a few weeks of the exact answer. Accuracy decreases slightly at very low or very high rates, where the Rule of 69.3 or Rule of 70 are sometimes used instead for better precision.
Can I use the Rule of 72 for debt instead of investments?
Yes — it works the same way for any compounding rate, including how long it takes a credit card balance to double at a given interest rate if left unpaid.
What's the exact formula the Rule of 72 approximates?
The precise doubling time is ln(2) / ln(1 + r), which involves natural logarithms; the Rule of 72 is a simplified linear approximation that's much easier to calculate mentally.
Why 72 and not another number?
72 divides evenly by many common interest rates (2, 3, 4, 6, 8, 9, 12), which makes the mental division simple, while still staying reasonably accurate across the typical range of investment returns.
Does the Rule of 72 work for inflation too?
Yes — it can estimate how long it takes prices to double at a given inflation rate, or equivalently, how long it takes purchasing power to halve.