What Is the Rule of 72?
The Rule of 72 is a simple, powerful mental math shortcut that estimates how long it takes for an investment to double in value. Divide the number 72 by your annual interest rate, and the result is the approximate number of years needed to double your money.
For example, at a 6% annual return, your investment doubles in approximately 12 years (72 ÷ 6 = 12). At 9%, it doubles in 8 years. This rule works best for interest rates between 4% and 12%, where it closely approximates the exact logarithmic formula.
Rule of 72 vs. Rule of 69.3
The mathematically precise version uses 69.3 (derived from the natural logarithm of 2, which is approximately 0.693). However, 72 is preferred in practice because it is divisible by more numbers (1, 2, 3, 4, 6, 8, 9, 12), making mental calculations much easier.
Practical Applications
- •Investment planning: Quickly compare different investment options without a calculator.
- •Debt management: Understand how quickly debt doubles if left unpaid. A 24% credit card doubles your balance in just 3 years.
- •Inflation impact: At 3% inflation, prices double in 24 years. At 6%, they double in just 12 years.