Process capability calculator (Cp, Cpk)
Calculate Cp, Cpk, Cpu and Cpl from the process mean, the standard deviation and the upper and lower specification limits.
Track capability over time
Need to follow this process after the one-off check? The SPC X-bar and R Control Chart Template helps you monitor the process with control charts.
How it is calculated
Cp = (USL − LSL) ÷ (6σ). Cpu = (USL − x̄) ÷ (3σ). Cpl = (x̄ − LSL) ÷ (3σ). Cpk = the lower of Cpu and Cpl.
Worked example
Mean 10.5, σ 0.5, limits 8 to 12: Cp = 4 ÷ 3 = 1.33, Cpu = 1.5 ÷ 1.5 = 1.00, Cpl = 2.5 ÷ 1.5 = 1.67, so Cpk = 1.00. The process is off-center, and Cpk shows it.
Steps
- Get the mean and standard deviation from a stable, in-control process.
- Enter the USL and LSL from the drawing or specification.
- Read Cp for spread alone and Cpk for spread plus centering.
Frequently asked questions
What is a good Cpk?
Many teams use 1.33 as the minimum for existing processes and 1.67 or more for safety-critical characteristics. Your customer may set a different target.
What is the difference between Cp and Cpk?
Cp measures spread only and ignores where the mean sits. Cpk also penalizes an off-center process, so Cpk is never higher than Cp.
Which standard deviation should I use?
Within-subgroup sigma gives Cp and Cpk. Overall sigma gives Pp and Ppk. Check which one your customer requires.
References
Montgomery, Introduction to Statistical Quality Control (process capability indices); AIAG Statistical Process Control (SPC) reference manual.
Estimates for information only. Verify critical values with a qualified engineer or your supplier standards.