What Is Process Capability?
Process capability measures whether your process can consistently produce output within customer specifications. It compares the voice of the process (natural variation) to the voice of the customer (specification limits). A capable process has variation that comfortably fits within the spec range.
This is the foundation of Six Sigma quality. If your process is not capable, no amount of inspection will fix it — you need to reduce variation or change the process.
Cp vs. Cpk: What They Mean
Cp (Capability Potential)
Can the process fit within the spec? Cp measures the width of the spec range relative to the width of the process spread (6 standard deviations). It assumes the process is perfectly centered.
Cp Formula
Cp = (USL – LSL) ÷ (6 × σ)
Where USL = Upper Spec Limit, LSL = Lower Spec Limit, σ = process standard deviation. A Cp of 1.0 means the process spread exactly equals the spec width — barely fits. A Cp of 2.0 means the spec is twice as wide as the process — comfortable room.
Cpk (Capability Performance)
Does the process actually fit? Cpk accounts for centering — how close the process mean is to the center of the spec. A process can have high Cp (narrow spread) but low Cpk (off-center, producing defects near one spec limit).
Cpk Formula
Cpk = min[(USL – μ) ÷ (3σ), (μ – LSL) ÷ (3σ)]
Where μ = process mean. Cpk takes the smaller of the two distances (mean to nearest spec limit) and divides by half the process spread. If Cpk = Cp, the process is perfectly centered. If Cpk < Cp, the process is off-center.
Interpreting the Numbers
| Cpk Value | Rating | Approx. DPMO | Sigma Level | What It Means |
|---|---|---|---|---|
| Below 0.67 | Not capable | 45,500+ | < 3σ | Process cannot meet spec. Major redesign or variation reduction needed. |
| 0.67 – 1.00 | Poor | 2,700 – 45,500 | 3-4σ | Significant defects. Process improvement urgent. |
| 1.00 – 1.33 | Marginal | 63 – 2,700 | 4-4.5σ | Producing some defects. Improvement needed for critical features. |
| 1.33 – 1.67 | Good | 0.6 – 63 | 4.5-5σ | Industry standard minimum for most manufacturing. Acceptable. |
| 1.67+ | Excellent | < 0.6 | 5+σ | Very capable. Variation well within limits. World class. |
| 2.00 | Six Sigma | 3.4 | 6σ | Six Sigma target. Near-zero defects. |
Visual: Capable vs. Not Capable
How to Improve Capability
✅ Capability-Driven Quality
- Cpk measured for every critical feature
- Process centered and variation reduced before production
- SPC charts monitored during production
- Capability studies required for new products/processes
❌ Inspection-Driven Quality
- Cpk unknown or never measured
- Rely on 100% inspection to sort good from bad
- No SPC — process drifts until defects appear
- New processes launched without capability study
🎯 Key Takeaway
Process capability tells you whether your process can meet spec consistently, not just whether today's parts are good. A Cpk of 1.33+ means you have breathing room; below 1.0 means you are producing defects by design. Measure capability for every critical feature, center the process, reduce variation, and monitor with SPC. It is the difference between hoping for quality and engineering it. Use the DPMO/Sigma calculator to connect capability to defect rates.
Interactive Demo
Adjust the process mean, standard deviation, and specification limits to see how Cp and Cpk change. Watch the distribution shift relative to the spec window.
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