Cp / Cpk Process Capability.
Paste up to 1000 measurements. Set spec limits. Get Cp, Cpk, Pp, Ppk, sigma level, and PPM defect rate with a live histogram.
1. Spec limits
2. Measurement data
Paste numbers separated by spaces, commas, tabs, or newlines.
| n (sample size) | — |
| Mean (X̄) | — |
| Std Dev (s) | — |
| Min · Max | — |
| Range | — |
Understanding Cp and Cpk: The Complete Guide
Cp and Cpk are process capability indices used to measure how well a manufacturing process can produce parts within specification limits. They tell you whether your process is capable of meeting customer requirements consistently — before you even ship a single defective part.
Cp vs Cpk Formula
Cp formula measures the spread of your process compared to the spec width:
Cp = (USL − LSL) / (6σ)
Where USL is the Upper Specification Limit, LSL is the Lower Specification Limit, and σ (sigma) is the process standard deviation.
Cpk formula accounts for both spread and centering — it tells you the worst-case capability based on which spec limit your process mean is closer to:
Cpk = min[(USL − μ)/(3σ), (μ − LSL)/(3σ)]
Where μ is the process mean. Cpk is always ≤ Cp — if they're equal, your process is perfectly centered.
Cp Cpk Worked Example
Suppose you're machining a shaft with a diameter spec of 10.00 ± 0.05 mm. After measuring 50 parts, you calculate mean μ = 10.005 mm, σ = 0.018 mm. Spec width is 0.10 mm.
- Cp = 0.10 / (6 × 0.018) = 0.93
- Cpk = min[(10.05 − 10.005)/(3 × 0.018), (10.005 − 9.95)/(3 × 0.018)] = min[0.83, 1.02] = 0.83
Both values are below 1.0 — the process is NOT capable. You'd expect significant out-of-spec parts. Either reduce variation (tighten σ) or improve centering, or both.
How to Interpret Cp and Cpk Values
- Cpk < 1.00 — Process NOT capable. High rejection risk. Stop and investigate.
- Cpk 1.00 – 1.33 — Marginally capable. Acceptable for non-critical features only.
- Cpk 1.33 – 1.67 — Capable. Industry-standard threshold per AIAG.
- Cpk ≥ 1.67 — Excellent (Six Sigma capable territory).
Most automotive customers (Tata, Mahindra, Bajaj) require Cpk ≥ 1.33 for PPAP submission. Aerospace customers often require ≥ 1.67 for critical characteristics.
When to Use Cp/Cpk vs Pp/Ppk
Use Cp/Cpk for short-term capability (within-subgroup variation only — your "best case" process performance). Use Pp/Ppk for long-term performance (overall variation including drift, shift change, batch variation).
Related Tools and Next Steps
After running a Cp/Cpk study, you'll typically need to verify your measurement system with Gauge R&R, set up control charts for ongoing monitoring, and document the capability in AS9102 Form 3 for FAI submission. For full PPAP packages, see the PPAP submission checklist.