One of the most useful mental math tricks in personal finance. No calculator needed — just divide 72 by your interest rate.
The Rule of 72 is the fastest way to estimate how long it takes for an investment to double at a given rate of return. It's not exact — but it's close enough to be genuinely useful, and you can do it in your head in seconds.
Years to Double ≈ 72 ÷ Annual Interest Rate (%)
That's it. If your investment earns 6% per year, it doubles in approximately 72 ÷ 6 = 12 years.
| Annual Return | Rule of 72 Estimate | Actual Years (exact) |
|---|---|---|
| 2% | 36 years | 35.0 years |
| 4% | 18 years | 17.7 years |
| 6% | 12 years | 11.9 years |
| 8% | 9 years | 9.0 years |
| 10% | 7.2 years | 7.3 years |
| 12% | 6 years | 6.1 years |
| 24% | 3 years | 3.2 years |
The Rule of 72 is most accurate between 6–10%, which happens to cover most long-term investment return expectations. The higher the rate, the slightly less accurate it becomes.
💡 Reverse it too: 72 ÷ Years = Required Rate. Want to double your money in 8 years? You need a 72 ÷ 8 = 9% annual return.
For an investment to double: (1 + r)t = 2. Solving for t using logarithms gives t = ln(2) / ln(1+r). For small r, ln(1+r) ≈ r, so t ≈ 0.693 / r. Since 0.693 ≈ 69.3, the mathematically precise version is actually the "Rule of 69.3" — but 72 divides evenly by more numbers (2, 3, 4, 6, 8, 9, 12), making mental math far easier.
When comparing two investments, the Rule of 72 makes the difference visceral:
The Rule of 72 works equally well for debt. A $10,000 credit card balance at 24% APR doubles to $20,000 in about 3 years if unpaid. At 18% APR: 4 years. This makes the urgency of paying off high-interest debt immediately obvious.
At 3% inflation, purchasing power halves in 24 years. At 6% inflation, it halves in 12. This is why earning a real return (above inflation) matters — money sitting in a 0% account loses half its value in a single generation at moderate inflation.
For precision, use a compound interest calculator with your actual numbers. The Rule of 72 is a mental shortcut — excellent for quick comparisons, terrible for contract negotiations or retirement planning where precision matters.
Get the exact doubling time — and a year-by-year growth chart — for any investment. Already know your target? Our savings goal calculator tells you exactly how much to save each month to hit it.
📈 Calculate Exact Compound GrowthThe Rule of 72 is one of those rare tools that takes 10 seconds to learn and pays dividends for the rest of your life. Memorize it. Use it to sanity-check any investment claim, understand the real cost of debt, and make the abstract concept of compounding concrete and intuitive.