Process Capability Calculator

How it works

1. 1Enter your Upper Specification Limit (USL), Lower Specification Limit (LSL), process mean (μ), and standard deviation (σ).
2. 2Cp is computed as (USL − LSL) / 6σ, reflecting the ratio of spec width to process spread assuming perfect centring.
3. 3Cpk is computed as min((USL − μ) / 3σ, (μ − LSL) / 3σ), penalising any shift of the mean away from the centre of the spec.

Use cases

• Manufacturing quality control — determining whether a machining process consistently produces parts within tolerance.
• Six Sigma projects — establishing a baseline capability before and after process improvements.
• Supplier qualification — verifying that a vendor’s process is capable of meeting your specification requirements.

Frequently asked questions

What is the difference between Cp and Cpk?

Cp measures potential capability — how wide the spec is relative to 6σ — assuming the process is perfectly centred. Cpk also accounts for how far the mean has drifted from the centre of the specification. A well-centred process has Cp ≈ Cpk; a shifted process has Cpk < Cp.

What Cpk value is considered acceptable?

Cpk ≥ 1.33 is the common industry benchmark for a capable process (equivalent to roughly 99.994% of parts within spec). Cpk between 1.0 and 1.33 is considered marginal, and Cpk below 1.0 indicates the process is not capable of reliably meeting the specification.

Can you show a worked example?

With USL = 10, LSL = 4, μ = 7, and σ = 1: Cp = (10 − 4) / (6 × 1) = 1.0 and Cpk = min((10 − 7) / 3, (7 − 4) / 3) = min(1.0, 1.0) = 1.0. The process is centred (Cp = Cpk) but only marginally capable — reducing σ or widening the spec would be needed to reach Cpk ≥ 1.33.