How do you calculate Cp?

Divide the tolerance width (USL minus LSL) by the six-sigma process spread. With a tolerance of 0.3 spec units and a spread of 0.24, Cp = 0.3 / 0.24 = 1.25, then multiplied by the conversion factor of 1 for a final Cp of 1.25.

What is a good Cp value?

A Cp of 1.33 is the common minimum for a capable process, and 1.67 or higher is preferred for critical characteristics. The 1.25 in this example is below the 1.33 threshold, indicating the process spread is a little wide relative to the tolerance.

What is the difference between Cp and Cpk?

Cp measures only the spread versus tolerance and assumes the process is perfectly centered, so it is the best-case potential. Cpk also accounts for how far the mean sits from the nearest spec limit, so Cpk is always less than or equal to Cp.

Why is the process spread six sigma wide?

A normal distribution places about 99.73% of output within plus or minus three sigma, a total width of six sigma. Using six sigma as the spread lets Cp express how many natural process widths fit inside the tolerance band.

What does a Cp below 1.0 mean?

It means the six-sigma spread is wider than the tolerance, so even a perfectly centered process will produce out-of-spec parts. You must reduce variation, not just re-center, to fix it.

What is the reporting conversion factor for?

It rescales the raw ratio when a customer or standard requires Cp expressed on a different basis or unit convention. Left at 1, it leaves the raw 1.25 ratio unchanged.

Quality & Metrology calculator

Process Capability Cp Calculator

Process capability Cp measures how well a process's natural variation fits inside the specification tolerance, ignoring where the process is centered. Quality engineers and SPC practitioners use it during process validation, PPAP submissions, and capability studies to answer a basic question: is the process precise enough for the spec, even in the best case? Cp is the ratio of the tolerance width to the six-sigma process spread, so a Cp of 1.0 means the spread exactly fills the tolerance with zero margin. Because Cp assumes perfect centering, it represents the potential capability, the ceiling that Cpk can never exceed.

What this calculator does

Estimate process capability Cp by comparing the tolerance width to the process spread for a feature.
Use it when you need a quick Cp on a characteristic and want to see how much of the tolerance the process is consuming.
It computes Cp as the tolerance width divided by the six-sigma process spread, optionally scaled by a reporting conversion factor.

Formula used

Cp = tolerance width ÷ process spread
Converted Cp = Cp × reporting conversion factor

Inputs explained

Tolerance width (USL minus LSL): Enter the total tolerance band, upper specification limit minus lower specification limit, for the feature.
Process spread (6 sigma): Enter six times the process standard deviation from a stable, in-control run of the same feature.
Reporting conversion factor: Leave at 1 unless you need to rescale the index into another basis; it does not change the capability ratio.

How to use the result

Use it during capability studies and process validation to judge whether process variation is tight enough for the specification, before worrying about centering.
Cp is blind to process centering, a process can show an excellent Cp while producing scrap because it is off-center, so always pair it with Cpk.

Current U.S. benchmarks

U.S. manufacturing runs at 76.0% of capacity (Federal Reserve, Jul 2026). New factory orders are up 7.4% year over year (Census).

Common questions

How do you calculate Cp? Divide the tolerance width (USL minus LSL) by the six-sigma process spread. With a tolerance of 0.3 spec units and a spread of 0.24, Cp = 0.3 / 0.24 = 1.25, then multiplied by the conversion factor of 1 for a final Cp of 1.25.
What is a good Cp value? A Cp of 1.33 is the common minimum for a capable process, and 1.67 or higher is preferred for critical characteristics. The 1.25 in this example is below the 1.33 threshold, indicating the process spread is a little wide relative to the tolerance.
What is the difference between Cp and Cpk? Cp measures only the spread versus tolerance and assumes the process is perfectly centered, so it is the best-case potential. Cpk also accounts for how far the mean sits from the nearest spec limit, so Cpk is always less than or equal to Cp.
Why is the process spread six sigma wide? A normal distribution places about 99.73% of output within plus or minus three sigma, a total width of six sigma. Using six sigma as the spread lets Cp express how many natural process widths fit inside the tolerance band.
What does a Cp below 1.0 mean? It means the six-sigma spread is wider than the tolerance, so even a perfectly centered process will produce out-of-spec parts. You must reduce variation, not just re-center, to fix it.

Last reviewed 2026-07-13.