Cp/Cpk Calculation Rule

For Cp
Cp = (USL - LSL) / (6 * StdDev)

For Cpk
CpkL = (Mean - LSL) / (3 * StdDev)
CpkU = (USL - Mean) / (3 * StdDev)
Cpk = Min(CpkL,CpkU)

Where: LSL = Lower Specification Limit
  USL = Upper Specification Limit