Process Capability Calculator

Cp, Cpk, Pp, Ppk, sigma level and estimated PPM. Deterministic, 100% client-side, no upload.

Inputs
Capability
—
Cp—
Cpk—
Pp—
Ppk—
Mean / Sigma—
Sigma level (≈3·Cpk)—
Est. out-of-spec PPM—

From ungrouped data, Pp/Ppk use the overall sample σ (n−1) and equal Cp/Cpk here. PPM assumes a normal distribution. Cpk ≥ 1.33 capable · 1.0–1.33 marginal · < 1.0 not capable.

About the Process Capability Calculator (Cp / Cpk / Pp / Ppk)

Statistical process control requires continuous evaluation of whether manufacturing lines can meet customer specifications. A process capability calculator computes critical indices like Cp, Cpk, Pp, and Ppk from sample data or known mean and standard deviation. By estimating Sigma levels and Parts Per Million (PPM) defect rates, quality engineers can proactively adjust machinery before generating nonconforming scrap.

How it works

  1. Input the Upper Specification Limit (USL) and Lower Specification Limit (LSL) provided by the customer.
  2. Either paste raw continuous measurement data or manually enter the process mean and standard deviation.
  3. Identify whether the standard deviation represents short-term (within-subgroup) or long-term (overall) variation.
  4. Review the calculated Cp, Cpk, Pp, Ppk indices and the estimated theoretical defect rate.

Frequently asked questions

What is the difference between Cp and Cpk?

Cp measures the potential capability of a process if it were perfectly centered between specification limits. Cpk measures the actual capability, penalizing the score if the process mean shifts closer to either specification limit.

What is the difference between Cpk and Ppk?

Cpk uses short-term standard deviation to measure process potential (what the process is capable of doing). Ppk uses long-term standard deviation to measure actual process performance over time, including external variations.

What is considered a good Cpk score?

A Cpk of 1.0 indicates the process mean is exactly 3 standard deviations from the nearest specification limit. The industry benchmark for a capable process is a minimum Cpk of 1.33 (4 sigma), while critical automotive or aerospace parts often require 1.67 (5 sigma) or 2.0 (6 sigma).

Why does the calculator estimate Parts Per Million (PPM)?

PPM translates abstract capability scores into tangible defect rates. For example, a Cpk of 1.0 implies a theoretical defect rate of approximately 2,700 parts per million, allowing management to estimate scrap costs.

References