What is the Rule of 72?
The Rule of 72 provides a quick way to estimate how long it takes for money to double. Simply divide 72 by the annual return rate. At 6% annual returns, your money doubles in approximately 12 years (72 / 6 = 12). At 8%, it doubles in about 9 years. At 3%, it takes 24 years.
Practical Applications
The rule works in reverse too. If prices double in 10 years, the annual inflation rate is roughly 7.2% (72 / 10). It is most accurate for rates between 4% and 12%. For a quick retirement calculation: if you need $1 million and have $250,000, you need your money to double twice - at 7% returns, that takes about 20 years.
Key Considerations
The Rule of 72 is an approximation that becomes less accurate at very high or very low rates. For rates above 20%, the Rule of 69.3 is more precise. Remember that the rule assumes compound interest and a constant rate of return, which real investments rarely provide. Use it for quick mental math, not precise financial planning.