How many parts is enough? What your Cpk really is
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.
Nothing you paste leaves this calculation. It is not stored, not logged and not sent anywhere else — the numbers are computed and returned, and that is all.
What your number really says
Interval by sample size
Why this matters
A capability index is an estimate from a sample, and it is a noisier estimate than its two decimal places suggest. Thirty pieces — the number most PPAP submissions use — leaves a Cpk of 1.33 with a 95% interval running from roughly 0.97 to 1.69. The same process, sampled again, could easily report 1.1 or 1.6.
That is the honest answer to "is thirty parts enough?": enough to estimate, not enough to prove a requirement of 1.33. To claim 1.33 with confidence you either need a much better process, or many more parts — this calculator gives you both numbers.
How this is calculated
SE(Ĉpk) ≈ √( 1/(9n) + Ĉpk² / (2(n−1)) ) CI = Ĉpk ± z × SE(Ĉpk)
The standard large-sample interval for Cpk (Bissell), the one behind the AIAG and Montgomery treatments. It assumes the data is roughly normal and — critically — that the process was in control while you sampled it.
The part most people get wrong
An unstable process has no capability. Sampling harder does not fix it: the interval narrows around a number that describes a process that no longer exists. Chart it first — that is what the control chart generator and the rules checker are for — and compute capability only over a stable stretch.
More parts is not the only lever. If the interval will not clear your customer's requirement at any sane sample size, the process needs to be better centred or less variable. No amount of measuring changes that.
Want this to keep itself up to date?
A calculator answers for the data you pasted. A control chart answers for the data your line produced this morning — limits frozen at a baseline you locked, rules evaluated on every new reading, an alert when one trips.
Other free tools
- 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.
- 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.
- 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.
- 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.
- 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.
- 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.