Process Capability Index Calculator

What Process Capability Index Measures

Process capability compares the natural spread of a process against specification limits. Manufacturers, laboratories, and quality teams use capability indices to see whether routine process output fits inside customer or engineering requirements.

Core Capability Formulas

$$C_p = \frac{USL - LSL}{6\sigma}$$

$$C_{pk} = \min\left(\frac{USL - \mu}{3\sigma}, \frac{\mu - LSL}{3\sigma}\right)$$

$C_p$ shows potential capability if the process were perfectly centered. $C_{pk}$ accounts for how far the actual process mean $\mu$ sits from each specification limit.

Target-Based Capability

Some quality studies care about a specific target, not just the nearest limit. In that case, you can also evaluate:

$$C_{pm} = \frac{C_p}{\sqrt{1 + \left(\frac{\mu-T}{\sigma}\right)^2}}$$

$$C_{pkm} = \frac{C_{pk}}{\sqrt{1 + \left(\frac{\mu-T}{\sigma}\right)^2}}$$

These indices penalize processes that drift away from target value $T$, even if they still remain inside the specification limits.

How To Interpret Cpk

• Below 1.00: process is generally not capable.
• 1.00 to 1.32: marginal capability.
• 1.33 to 1.99: capable process.
• 2.00 or above: excellent capability.

Related Quality Tools

Capability analysis works best alongside data-quality checks such as our Standard Deviation Calculator, Upper Control Limit Calculator, and Normal Distribution Calculator.