Process Capability Index Calculator

Calculate Cp, Cpk, Cpm, and Cpkm indices for quality control

Process Parameters

Maximum acceptable value

Minimum acceptable value

Average value of the process

Process variability (must be positive)

0

Example Calculation

Soup Temperature Control Example

Specification: Soup temperature must be 64-80°C

Process mean: 72°C

Standard deviation: 1.5°C

Calculation Results

Cp: (80 - 64) / (6 × 1.5) = 16 / 9 = 1.778

CPU: (80 - 72) / (3 × 1.5) = 8 / 4.5 = 1.778

CPL: (72 - 64) / (3 × 1.5) = 8 / 4.5 = 1.778

Cpk: min(1.778, 1.778) = 1.778

Result: Excellent process capability (Cpk > 1.33)

Capability Interpretation

< 0.67 - Poor

Process is not capable

0.67-1.00 - Inadequate

Process needs improvement

1.00-1.33 - Adequate

Marginally capable

1.33-2.00 - Good

Process is capable

> 2.00 - Excellent

World-class (6-sigma)

Key Concepts

📊

Cp: Potential capability assuming perfect centering

🎯

Cpk: Actual capability considering process centering

📍

Cpm: Capability relative to target value

Cpk ≥ 1.33: Industry standard for capable process

Understanding Process Capability Index

What is Process Capability?

Process capability indices measure how well a process can produce output within specification limits. They compare the width of the process distribution to the width of the specification limits, helping determine if a process is capable of meeting customer requirements.

Types of Indices

  • Cp: Measures potential capability if process is perfectly centered
  • Cpk: Measures actual capability considering process centering
  • Cpm: Capability relative to a specific target value
  • Cpkm: Combined measure of centering and target deviation

Formula Explanations

Cp = (USL - LSL) / (6σ)

Cpk = min[(USL - μ)/(3σ), (μ - LSL)/(3σ)]

Cpm = Cp / √[1 + ((μ - T)/σ)²]

Cpkm = Cpk / √[1 + ((μ - T)/σ)²]

  • USL: Upper Specification Limit
  • LSL: Lower Specification Limit
  • μ: Process Mean
  • σ: Standard Deviation
  • T: Target Value

Industry Standards & Applications

Manufacturing

Quality control in production processes, ensuring products meet dimensional and performance specifications.

Six Sigma

Cpk ≥ 2.0 indicates 6-sigma quality level with 3.4 defects per million opportunities (DPMO).

Service Industries

Service time, quality metrics, and customer satisfaction measurements in service processes.