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SYMESTIC CP/CPK CALCULATOR

Process Capability Calculator (Cp / Cpk)

Calculate Cp, Cpk, Pp, Ppk, the expected defect rate (ppm) and the yield of your process – including the bell curve with specification limits and a centering analysis.

Guide FAQ
Input mode
Enter the mean and standard deviation directly – or paste your measurement series and let both be calculated.
Specification limits
Tolerance range of the characteristic. A single-sided USL or LSL is enough – only the corresponding index is then calculated.
Upper specification limit (USL)
Upper tolerance value
Lower specification limit (LSL)
Lower tolerance value
Nominal / target (optional)
For display in the bell curve only
Process data
Mean and sample standard deviation (s, divisor n−1) of your process.
Mean (x̄)
Process location
Standard deviation (s)
Sample standard deviation (n−1)
Measured values
Decimal separator: period. Separate values with a comma, space or line break. At least 2 values.
Number of values (n) –
Mean (x̄) –
Standard deviation (s) –
CPK
Rating
Excellent

Cp (potential)
–
Defects
–ppm
Distribution & specification limits
Normal distribution of the process with mean, USL and LSL. The red areas mark the expected share of defects outside the tolerance.
Process distribution
Defect share (outside tolerance)
The curve is a normal distribution built from your mean and standard deviation. If the mean does not sit centered between the limits, the process is not centered.
Cp (potential)
–
Tolerance width ÷ 6σ · pure spread potential without location
Cpu (upper)
–
(USL − x̄) ÷ 3σ · distance to the upper limit
Ppk (long-term)
–
Long-term reading · equal to Cpk here (σ from total spread)
Yield
–%
Share of parts within the tolerance
Centering: Cp vs. Cpk
Centered
Cp is the potential (spread only), Cpk is the actual value (spread + location). A large difference indicates an off-center process location.
Cp (potential)
–
Cpk (actual)
–
i
Classifying the Cpk value
Common rating bands for process capability.
–
Not capable < 1.00
Marginal 1.00–1.33
Capable 1.33–1.67
Excellent ≥ 1.67
Binding target values depend on the industry and customer (automotive often 1.33 or 1.67). We make no claim of industry-specific target figures.
Recommendations
Maintain level
Maintain & monitor capability
The process is capable. Lock in the level with statistical process control (SPC), control charts and preventive equipment maintenance.
Tool: OEE Calculator
Primary lever
Center the process
Cp is significantly larger than Cpk – the mean sits off-center. Align the process to the nominal value (tool correction, setting parameters). This raises Cpk substantially without reducing spread.
Tool: Scrap Rate / FPY Calculator
Primary lever
Reduce spread
Cp and Cpk are similarly low – the process spread (6σ) is too large for the tolerance. Reduce the spread through machine capability, material and method stability (SPC, control charts, DoE).
Tool: Scrap Rate / FPY Calculator
Share & export your result
Your inputs are stored in the link – no account, no backend.
Request a live demo

Enter at least one specification limit plus the mean and standard deviation (or measured values) to calculate Cp, Cpk and the defect rate.

A free tool from SYMESTIC – Manufacturing Execution System from the cloud.
HOW THE CALCULATOR WORKS

Quick guide: calculate Cp & Cpk in 3 steps

From specification limits and process location, the tool instantly calculates Cp, Cpk, Pp, Ppk, ppm and yield. Here's how:

1

Enter the specification limits

Enter the upper (USL) and/or lower (LSL) tolerance limit. Optionally the nominal value for display in the bell curve. Single-sided tolerances are allowed.

2

Capture the process location

Enter the mean and standard deviation directly – or switch to "Raw data" in the top right and paste your measurement series. The tool calculates x̄ and s.

3

Read & interpret the result

Cp, Cpk, ppm and yield appear instantly. The bell curve shows the location relative to the tolerance, and the centering analysis shows the biggest improvement lever.

The formulas at a glance
Cp (USL − LSL) ÷ 6σ Process potential – the ratio of tolerance width to spread, without accounting for location.
Cpu / Cpl (USL−x̄)÷3σ · (x̄−LSL)÷3σ Single-sided capability indices toward the upper and lower limit respectively.
Cpk min(Cpu, Cpl) Actual capability – the more critical distance to the nearest specification limit.
Pp / Ppk = Cp / Cpk Long-term performance using total spread. With this σ source they are mathematically identical to Cp/Cpk.
ppm 10⁶ × (Φ_lower + Φ_upper) Expected defect share per million outside the specification limits under a normal distribution.
σ here is the sample standard deviation (divisor n−1) across all measured values. ppm is calculated assuming a normal distribution (Φ = standard normal distribution).
FREQUENTLY ASKED QUESTIONS

Cp, Cpk & process capability – frequently asked questions

Terms, formulas and practical tips around the capability of your processes.