Free SPC tools
The calculations quality engineers do every week, on a page instead of in a spreadsheet. No signup, no email, nothing you paste is stored — and the maths is the same tested engine that draws the charts in the product.
Cp / Cpk calculator
Paste measurements — or type a mean and a sigma — with your tolerance, and get Cp, Cpk, Pp, Ppk, the sigma level and the expected parts per million out of spec.
Open →Control limit calculator
Give it subgroups or individual readings and it returns the control limits for the chart, the range or sigma chart beneath it, and every constant it used to get there.
Open →Control chart generator
Paste a column of numbers, or rows of subgroups, and get a real control chart: limits from the data, Nelson rules 1–4 evaluated, out-of-control points marked.
Open →Nelson & Western Electric rules checker
Every run rule evaluated on your data, each violation named in plain English with what it usually indicates — a shift, a trend, tool wear, two machines mixed.
Open →Cpk confidence interval & sample size
A Cpk of 1.33 from 30 pieces has a 95% interval of roughly 0.97 to 1.69. See the uncertainty in your own number, and how many parts would settle it.
Open →Cpk ↔ PPM ↔ sigma level converter
What PPM is a Cpk of 1.33? What Cpk does 3.4 PPM need? Both conventions shown side by side, because the 1.5 sigma shift is why two sources disagree by a factor of ten.
Open →Control chart constants
The whole table, n = 2 to 25, with the formula each constant belongs to. The same values the charts on this site are computed from.
Open →Want any of this to keep itself up to date from the line instead? See it running on live public data, or start a free trial.