What is the Rule of 72
The Rule of 72 is a mental math shortcut for estimating how long it takes an investment to double — divide 72 by the annual interest rate or return, and the result is the approximate number of years to double your money.
The Rule of 72 is a simple mental math formula for estimating how long it takes for an investment to double in value at a given annual rate of return. Divide 72 by the annual return percentage, and the result is the approximate number of years to double your money.
Formula:
Years to double ≈ 72 ÷ Annual Rate of Return (%)
Examples
| Annual Return | Years to Double (Rule of 72) | Actual Years |
|---|---|---|
| 2% | 72 ÷ 2 = 36 years | 35.0 years |
| 4% | 72 ÷ 4 = 18 years | 17.7 years |
| 6% | 72 ÷ 6 = 12 years | 11.9 years |
| 8% | 72 ÷ 8 = 9 years | 9.0 years |
| 10% | 72 ÷ 10 = 7.2 years | 7.3 years |
| 12% | 72 ÷ 12 = 6 years | 6.1 years |
| 18% | 72 ÷ 18 = 4 years | 4.2 years |
| 36% | 72 ÷ 36 = 2 years | 2.3 years |
The rule is accurate within 1–2% for rates between 6–10%, and reasonably close for rates from 2–20%.
Why 72?
The "exact" number for the doubling formula based on natural logarithm math is 69.3 (since $\ln(2) \approx 0.693$). The number 72 is used instead because:
- It's close to 69.3 in value
- It has more factors (1, 2, 3, 4, 6, 8, 9, 12, 24, 36, 72) — making division easy for common interest rates
- 72 ÷ 6 = 12; 72 ÷ 8 = 9; 72 ÷ 9 = 8 — all whole numbers, making quick mental math easy
Practical Applications
Investment planning: At a 7% average stock market return (roughly the historical inflation-adjusted return of a diversified index fund), your money doubles approximately every 10 years. Over 40 years, money doubles four times: $10,000 → $20,000 → $40,000 → $80,000 → $160,000.
Understanding inflation: The rule works in reverse for inflation. At 3% inflation, purchasing power halves in 24 years (72 ÷ 3). At 7% inflation, it halves in ~10 years — visualizing why high inflation is devastating to savings.
Comparing savings accounts: A high-yield savings at 5% doubles money in 14.4 years; a regular bank savings at 0.1% takes 720 years. Illustrates the cost of leaving money in a low-yield account.
Compound interest illustration: The Rule of 72 is most powerful as a way to viscerally feel the impact of compound interest over time — not just linear growth, but exponential doubling.
Debt: The rule applies to debt too. A credit card at 24% APR: any balance you don't pay off doubles in 3 years. A student loan at 6%: the outstanding balance doubles every 12 years if unpaid.
Related Rules
Rule of 114: Divide 114 by the rate to find how long it takes money to triple.
Rule of 144: Divide 144 by the rate to find how long it takes money to quadruple.
These are less commonly used but follow the same logic — useful for understanding multi-generational wealth compounding.
The Rule of 72 is one of the most practical and memorable tools in personal finance — a 3-second mental calculation that helps evaluate investment decisions, understand the impact of fees and inflation, and grasp the power of compound interest in concrete, intuitive terms.