What Is the Rule of 72?
The Rule of 72 is a simple mental math shortcut that estimates how long it takes for an investment to double in value at a given annual rate of return. Simply divide 72 by the annual return percentage to get the approximate number of years to double. At 6% annual return, your money doubles in approximately 72/6 = 12 years. At 12% return, it doubles in about 72/12 = 6 years. This rule is remarkably accurate for interest rates between 4% and 15% and provides a quick way to evaluate investments and understand the power of compound growth.
The Rule of 72 can also be used in reverse: to find the rate of return needed to double your money in a specific number of years, divide 72 by the number of years. To double your money in 8 years, you need a return of 72/8 = 9% per year. This makes the rule useful for quickly evaluating whether an investment's expected return meets your goals.
To estimate tripling time, use the Rule of 114 (divide 114 by the return). To estimate quadrupling, use the Rule of 144 (divide 144 by return, or simply double the doubling time). At 8%, money doubles in 9 years, triples in 14.25 years, and quadruples in 18 years.
The Rule of 72 Formula
| Annual Return | Rule of 72 Estimate | Exact Doubling Time | Difference |
|---|---|---|---|
| 4% | 18.0 years | 17.67 years | 0.33 years |
| 6% | 12.0 years | 11.90 years | 0.10 years |
| 7% | 10.3 years | 10.24 years | 0.06 years |
| 8% | 9.0 years | 9.01 years | 0.01 years |
| 10% | 7.2 years | 7.27 years | 0.07 years |
| 12% | 6.0 years | 6.12 years | 0.12 years |
| 15% | 4.8 years | 4.96 years | 0.16 years |
- 1Double 1: $10,000 → $20,000 in 72/8 = 9 years
- 2Double 2: $20,000 → $40,000 in 9 more years (18 total)
- 3Double 3: $40,000 → $80,000 in 9 more years (27 total)
- 4Double 4: $80,000 → $160,000 in 9 more years (36 total)
- 5Reverse: To double in 6 years, need 72/6 = 12% return
- 6Inflation check: At 3% inflation, purchasing power halves in 72/3 = 24 years
Practical Applications of the Rule of 72
- Investment evaluation: Quickly assess if an investment meets your growth expectations
- Retirement planning: Estimate how many times your money will double before retirement
- Inflation impact: At 3% inflation, prices double every 24 years (72/3)
- Credit card debt: At 18% interest, unpaid credit card debt doubles every 4 years (72/18)
- GDP growth: A country growing at 4% doubles its economy every 18 years
- Population growth: At 1.1% growth, world population doubles every 65 years
- Comparing investments: Quickly see which investment doubles faster
Related Rules
Extended Doubling Rules
Rule of 72 for Canadian Investors
The Rule of 72 applies universally regardless of currency or country. Canadian investors can use it for TFSA growth estimates (7% return = doubles every 10.3 years), RRSP projections, GIC comparisons, and inflation impact on Canadian dollar purchasing power. With Bank of Canada targeting 2% inflation, Canadian purchasing power halves every 36 years (72/2). At a typical balanced portfolio return of 6-7%, Canadian investments double in 10-12 years. The rule is particularly useful for quick comparisons between HISA rates (4-5%), GIC rates (4-5%), and equity returns (7-10%).
The Rule of 72 is an approximation that works best for rates between 4-15%. At very low rates (1-2%) or very high rates (20%+), the approximation becomes less accurate. It also assumes a constant rate of return, which does not reflect real-world investment volatility. Use it for quick estimates and mental math, but use proper compound interest calculators for precise financial planning.