To calculate the exact number of years required to double an investment at a constant 7 percent annual compound interest rate, financial planners apply the Rule of 72. By dividing the number 72 by the annual interest percentage rate of 7, the mathematical result is approximately 10.28 years. This means that if you invest a lump sum into an asset compounding continuously at a steady 7 percent rate of return, your initial principal investment will grow to twice its original monetary value in just over ten years, assuming no intermediate withdrawals or management fee deductions.