Cp is an abbreviation. There are really two parts, the upper and the lower denoted Cpu and Cpl, respectively. Their equations are: Cpl = (Process Mean – LSL)/ (3*Standard Deviation) Cpu = (USL – Process Mean)/ (3*Standard Deviation) Cpk is merely the smallest value of the Cpl or Cpu denoted: Cpk = Min (Cpl, Cpu)

The value of Cpk is the minimum of two process capability indices. One process capability is Cpu, which is the process capability based on the upper specification limit. The other is Cpl, which is the process capability based on the lower specification limit. Algebraically, Cpk is defined as: Cpk = Minimum (Cpu, Cpl)

Cpu = (Process mean - LSL)/ (3 * Standard deviation) where the LSL is the lower specification limit. Cpl = (USL - Process Mean)/ (3 * Standard deviation) where the USL is the upper specification limit. Now the term Cpk is only the value that is smallest of the above two, that is Cpu and Cpl.

Cpk is a measure of the potential capability of the process and equals the minimum of CPU and CPL. Cpk is a ratio that compares two values: The distance from the process mean to the closest specification limit (USL or LSL) The one-sided spread of the process (the 3-σ variation) based on the within-subgroup standard deviation