How do you calculate Cpk?

Subtract any required clearance buffer from the distance between the mean and the nearest spec limit, then divide by the one-sided three-sigma spread. With a distance of 0.4, a 0.04 buffer, and a 0.27 three-sigma spread, the effective distance is 0.36 and Cpk = 0.36 / 0.27 = 1.33.

What is a good Cpk value?

1.33 is the standard minimum for a capable process, and 1.67 is expected for safety-critical or high-volume characteristics. The 1.33 in this example sits right at the common acceptance threshold.

Cpk vs Cp — which should I report?

Report both. Cp is the potential if perfectly centered; Cpk is the actual capability given current centering. If Cpk is much lower than Cp, the fastest improvement is re-centering the process, not reducing variation.

Why use the three-sigma spread instead of six-sigma?

Cpk is one-sided — it looks only at the distance to the nearest limit — so it compares against the one-sided three-sigma spread rather than the full six-sigma width used by Cp.

What does the clearance buffer do to Cpk?

The buffer reserves margin against the spec limit, reducing the effective distance to the limit before the ratio is taken. Here, the 0.04 buffer trims 0.4 down to 0.36, which lowers Cpk to keep a safety cushion.

Yes. If the process mean sits outside the spec limit, the distance to the limit goes negative and so does Cpk, meaning more than half the output is already out of spec.

Quality & Metrology calculator

Process Capability Cpk Calculator

Process capability Cpk measures how well a process meets specification while accounting for where it is actually centered, making it the realistic counterpart to the best-case Cp. Quality engineers rely on it during validation, supplier qualification, and ongoing SPC because it captures the true risk of producing out-of-spec parts on the side where the process runs closest to the limit. Cpk compares the distance from the process mean to the nearest spec limit against the one-sided three-sigma spread. A Cpk below Cp signals the process is off-center, and the gap between them tells you exactly how much yield you are leaving on the table to poor centering.

What this calculator does

Estimate process capability Cpk as the distance from the process mean to the nearest specification limit, divided by the one-sided process spread.
Use it when a feature may be off-center and you need a capability number that reflects the worst-case spec limit for a go or no-go review.
It computes Cpk as the centering-adjusted distance to the nearest spec limit divided by the one-sided three-sigma process spread.

Formula used

Adjusted distance to limit = distance to nearest spec limit − required clearance buffer
Cpk = adjusted distance to limit ÷ one-sided process spread (3 sigma)

Inputs explained

Distance from mean to nearest spec limit: Enter how far the process mean sits from the closer specification limit, USL or LSL.
Required clearance buffer: Enter any margin you want to reserve before the limit for drift; leave at 0 to read raw Cpk.
One-sided process spread (3 sigma): Enter three times the process standard deviation from a stable, in-control run of the feature.

How to use the result

Use it whenever you need the realistic capability of a process that may be off-center, especially for one-sided specs or critical characteristics.
Cpk reflects only the worst side and a single moment in time; it does not capture long-term drift between subgroups, which Ppk and control charts address.

Current U.S. benchmarks

U.S. manufacturing runs at 75.6% of capacity (Federal Reserve, May 2026). New factory orders are up 2.3% year over year (Census).

Common questions

How do you calculate Cpk? Subtract any required clearance buffer from the distance between the mean and the nearest spec limit, then divide by the one-sided three-sigma spread. With a distance of 0.4, a 0.04 buffer, and a 0.27 three-sigma spread, the effective distance is 0.36 and Cpk = 0.36 / 0.27 = 1.33.
What is a good Cpk value? 1.33 is the standard minimum for a capable process, and 1.67 is expected for safety-critical or high-volume characteristics. The 1.33 in this example sits right at the common acceptance threshold.
Cpk vs Cp — which should I report? Report both. Cp is the potential if perfectly centered; Cpk is the actual capability given current centering. If Cpk is much lower than Cp, the fastest improvement is re-centering the process, not reducing variation.
Why use the three-sigma spread instead of six-sigma? Cpk is one-sided — it looks only at the distance to the nearest limit — so it compares against the one-sided three-sigma spread rather than the full six-sigma width used by Cp.
What does the clearance buffer do to Cpk? The buffer reserves margin against the spec limit, reducing the effective distance to the limit before the ratio is taken. Here, the 0.04 buffer trims 0.4 down to 0.36, which lowers Cpk to keep a safety cushion.

Last reviewed 2026-05-12.