Question: what is the mathematical equation for the rule of 72?
Answer: For example, the Rule of 72 states that $1 invested at an annual fixed interest rate of 10% would take 7.2 years ((72/10) = 7.2) to grow to $2. In reality, a 10% investment will take 7.3 years to double ((1.107.3 = 2). The Rule of 72 is reasonably accurate for low rates of return.