These calculators compute the process capability index (Cp, Cpk) which shows the process potential of meeting the specifications. Enter the process parameters and specifications in one of the following tables, depending on whether you have a double-sided or single-sided specification.
Historical note: "Six sigma" really started with Cpk... Motorola wanted to tolerate process shifts (i.e., changes in the process mean) of up to 1.5σ in either direction but still wanted to ensure a low defect rate. If the USL and LSL are both set to be 6σ (six sigma!) away from the mean, then a maximal process shift of 1.5σ would still produce the magical number of 3.4 defective parts per millions (PPM).