Quality & Statistics
Process Capability Calculator
Cp, Cpk, PPM defect rate, and sigma level from your process data or summary statistics.
Cpk = min(Cpu, Cpl)What is process capability?
Process capability measures how well a process meets specification limits. It compares the voice of the process (actual variation) to the voice of the customer (spec limits). A capable process consistently produces output within specifications.
- Cp — potential capability (spread only)
- Cpk — actual capability (spread + centering)
- Pp / Ppk — performance indices (overall variation)
Key formulas
Potential capability
Cp = (USL − LSL) ÷ (6σ)Upper / lower capability
Cpu = (USL − x̄) ÷ (3σ) · Cpl = (x̄ − LSL) ÷ (3σ)Actual capability
Cpk = min(Cpu, Cpl)Cpk benchmarks
| Cpk | Sigma level | Long-term PPM (1.5σ shift) | Rating |
|---|---|---|---|
| 2.00 | 6σ | 3.4 | World class |
| 1.67 | 5σ | 233 | Excellent |
| 1.33 | 4σ | 6,210 | Good |
| 1.00 | 3σ | 66,807 | Barely capable |
| < 1.00 | < 3σ | > 66,807 | Not capable |
PPM figures follow the Six Sigma convention that includes a 1.5σ long-term shift. The calculator reports the PPM expected from your mean and σ with no shift, so a centred process at Cpk 1.33 shows about 63 PPM there.
Frequently asked questions
What is the difference between Cp and Cpk?
Cp measures potential capability assuming the process is centered. Cpk accounts for how far the process mean is from the center of the specification. If Cp is much larger than Cpk, the process is off-centre — re-centring it would recover most of the gap.
What Cpk should I target?
Most industries require Cpk ≥ 1.33 for existing processes and Cpk ≥ 1.67 for new processes. Automotive (IATF 16949) and aerospace often require higher values. Six Sigma targets Cpk = 2.0.
What is the difference between Cpk and Ppk?
Cpk uses within-subgroup standard deviation (short-term variation), while Ppk uses overall standard deviation (long-term variation including between-subgroup variation). If Cpk ≫ Ppk, there is significant between-subgroup variation that needs to be addressed.