Cpk = 1.00: The Bare-Minimum Capability

Why is Cpk = 1.00 not a passing state, and what specifically is at risk?

Three standard deviations sit exactly on each spec limit. There is zero safety margin. In real production this state is almost always temporary — it drifts.

• Typical PPM: ≈ 1,350 PPM at the nearest limit; ≈ 2,700 PPM only for a perfectly centered, two-sided process
• Sigma equivalent: ≈ 3σ

Common automotive practice uses 1.33 for established production and 1.67 for some initial or launch studies, but the controlling value is always the applicable customer-specific requirement, drawing, Control Plan, or internal standard. Do not treat these values as universal pass/fail rules.

1. Treat Cpk = 1.00 as red on the scorecard, not amber.
2. Verify Cp; if Cp is also near 1.00, variation is the ceiling.
3. Model expected PPM at Cpk 0.90 and 0.80 to size the containment risk if drift occurs.
4. Plan the improvement path (variation reduction or re-centering) to reach the customer target.

• Optimistic rounding — Cpk 0.98 rounded to 1.00 in a template.
• Confusing "meets minimum" with "acceptable"; most customers require margin above 1.

• Cpk = 1.00 corresponds to 3σ at the nearest limit; the sigma level equals the index times three.
• In real production, any thermal, tool-wear, or material variation reliably pushes Cpk under 1 within one tool life.
• Reporting Cpk = 1.00 to a customer expecting 1.33 without a stated recovery plan is a scorecard failure.

• Do not use Cpk = 1.00 as evidence of a healthy launch — it is the mathematical boundary, not a target.

• Cpk 1.00 ≈ 3σ ≈ 1,350 PPM at the nearest tail; ≈ 2,700 PPM total only for a centered two-sided process.
• To reach Cpk 1.33 from 1.00 requires either σ reduction of ~25% or a centering improvement, given constant Cp.

1. Treat Cpk = 1.00 as 'red'. It is not a pass state.
2. Target Cpk ≥ 1.33 minimum, ≥ 1.67 for PPAP.
3. Use the free Process Capability Calculator to compute Cp, Cpk, Pp, Ppk, sigma level, and predicted PPM from your measurement data — no sign-up required.

• Never plan production capacity assuming Cpk stays exactly at 1.00 — it will drift.

• ✓Governing target (1.33, 1.67, or other) confirmed from CSR / drawing
• ✓Gap to target quantified in σ, PPM, and cost of poor quality
• ✓Improvement plan classified as centering, variation, or design

• Process condition: statistical stability is required.
• Distribution assumption: approximately normal data.
• PPM convention: nearest specification-limit tail.
• Sigma-shift convention: no sigma shift applied.
• Cpk alone identifies distance to the nearest specification limit; it does not determine total two-sided defects unless centering or both tail distances are known.
• Predicted PPM is a model estimate, not a replacement for observed defect data.