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Process Capability
Upload raw measurements to calculate Cp, Cpk, and dynamically chart your process distribution against spec limits.
Process Data
Auto Fill Example
Copy a column from Excel/Sheets. Separated by spaces, commas, or newlines.
0 values parsed
Specifications
The minimum acceptable value.
The maximum acceptable value.
The ideal value you aim for.
Process Analysis
Ready to Analyze
Enter your measurements and spec limits on the left or use an Auto Example to generate the capability report.
View Formula & Math
View Formula & Math
Standard Formulas
Process Capability Potential (Cp)
Cp = (USL - LSL) / 6σ
Process Capability Index (Cpk)
Cpk = min( (USL-Mean)/3σ, (Mean-LSL)/3σ )
Sigma Estimate
Sigma = 3 × Cpk + 1.5
What is Process Capability (Cp & Cpk)?
Process Capability is a statistical measurement of how well a manufacturing or business process meets its design specifications or customer requirements. It compares the inherent variation of a stable process against the acceptable tolerance limits (Upper and Lower Specification Limits).
Cp vs Cpk: What's the Difference?
It's critical to measure both Cp (Potential Capability) and Cpk (Actual Capability) to fully understand your process health.
- Cp (Process Capability Index): Measures the spread of your process relative to the specification width. It answers: "Could this process fit within the limits if it were perfectly centered?"
- Cpk (Process Capability Index, Centered): Measures both the spread and the centering of the process. It answers: "How well is this process actually performing right now?"
Rule of thumb: If Cp is high but Cpk is low, your process is consistent but off-target. You just need to re-center it. If Cp is low, your process is too variable, regardless of centering.
How to use the Capability Calculator
- USL & LSL: Enter the Upper and Lower Specification Limits defined by your engineering team or customer.
- Mean (Average): Enter the calculated average of your measured data points.
- Standard Deviation: Enter the standard deviation of your sample data. (Make sure your data is normally distributed).
The calculator will automatically determine your Cp, Cpk, Cpl (lower capability), and Cpu (upper capability), alongside an interpretation of your process Zbench (sigma distance).
Frequently Asked Questions
What is a good Cpk value?
Generally, a Cpk of 1.33 is considered the industry minimum standard (equivalent to 4 Sigma). A Cpk of 1.67 or higher is considered excellent (5 Sigma), and 2.0 is world-class (6 Sigma).
Can Cpk be negative?
Yes, Cpk can be negative if the process mean falls outside the specification limits. This indicates a severely flawed process where more than 50% of the output is defective.