Process Capability Calculator (Cp / Cpk)
Check whether your process fits its tolerance: enter the spec limits, mean and standard deviation to get Cp and Cpk. Free, instant and fully in your browser.
Cpk
1.333
Cp = (USL − LSL) / 6σ · Cpk = min(Cpu, Cpl)
Cp ignores centring (potential capability); Cpk also penalises an off-centre process (actual capability). A common target is Cpk ≥ 1.33. Standard deviation must be greater than zero.
What is process capability (Cp / Cpk)?
Process capability compares the spread of a process to the width of its specification. Cp measures the potential capability — how the natural variation (6σ) fits inside the tolerance — assuming the process is perfectly centred. Cpk goes further and accounts for where the process is actually centred, taking the worse of the upper and lower margins. A capable, centred process has Cp equal to Cpk; when the two diverge, the process is drifting toward one limit and needs re-centring.
How to calculate Cp and Cpk
- 1
Set the specification
Enter the upper and lower specification limits (USL and LSL) the product must fall between.
- 2
Measure the process
From a stable sample, enter the process mean (µ) and the standard deviation (σ) of the characteristic.
- 3
Read Cp and Cpk
Cp = (USL − LSL) / 6σ for potential capability; Cpk = min((USL − µ)/3σ, (µ − LSL)/3σ) for actual capability.
Frequently asked questions
What is a good Cpk value?
A widely used minimum is Cpk ≥ 1.33, which corresponds to a comfortably capable process; many industries target ≥ 1.67. Below 1.0 the process is producing out-of-spec output and needs attention.
Why is Cpk lower than Cp?
Cp assumes the process is centred between the limits. Cpk uses the nearer limit, so any shift off centre lowers Cpk while leaving Cp unchanged. A large gap means re-centring the process can recover capability without reducing variation.
How does Yamazo Studio help?
Capability requires stable, standardised work. Yamazo Studio turns shop-floor video into standard work and motion analysis that reduce variation at the source — the foundation of a capable process.
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