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Cpk calculator online

Paste your measurements and spec limits — instantly see Cpk, Cpu, Cpl, Cp and a verdict against IATF 16949 (baseline 1.33) and AIAG-VDA safety characteristic threshold (1.67). No Excel, no add-in, no waiting for IT.

Your data never leaves the browser — all computation is client-side.

Paste numbers separated by comma, space, tab or newline. At least 2 values (30+ recommended for a stable σ estimate).

Maximum allowed value

Minimum allowed value

How Cpk is computed and what it means

Short theoretical reference for anyone who wants to verify the result in Excel or by hand.

Formula

Cp  = (USL − LSL) / (6σ)
Cpu = (USL − x̄)  / (3σ)
Cpl = (x̄  − LSL) / (3σ)
Cpk = min(Cpu, Cpl)

x̄   = sample mean
σ   = sample standard deviation (n − 1)

Interpretation per IATF + AIAG-VDA

  • Cpk < 1.00: process is not capable — produces parts out of spec. Containment + root cause + immediate actions.
  • 1.00 ≤ Cpk < 1.33: marginal. Sufficient for normal characteristics, but the IATF baseline is 1.33 — improve before PPAP submission.
  • 1.33 ≤ Cpk < 1.67: IATF baseline for common characteristics. PPAP-acceptable at most OEMs.
  • Cpk ≥ 1.67: Safety / Critical Characteristics (CC, SC) per AIAG-VDA + customer CSR (VW Formel Q, Stellantis SQAM, Ford Q1, etc.).

Cpk vs Ppk — short

Cpk uses within-subgroup sigma (estimated via R̄/d₂ from rational subgroups) — it measures potential capability under a stable process. Ppk uses overall sigma across all measurements — it measures actual performance including between-subgroup variability. This calculator uses overall sigma — technically the result is Ppk even though we label it Cpk for practical clarity. For strict within-subgroup Cpk (Xbar-R, Xbar-S charts) you need to specify the subgroup size — we will add that when real customers ask.

Frequently asked questions

How many measurements do I need for a reliable Cpk?
Mathematical minimum is 2 (needed to compute standard deviation). For a stable σ estimate the AIAG SPC Manual recommends at least 30 measurements. PPAP submission to VW, Stellantis, BMW, Ford typically requires a minimum of 30 — some require 50 or 100 for safety characteristics. Always check the customer CSR or your Control Plan.
What is the difference between Cp and Cpk?
Cp = (USL − LSL) / (6σ) ignores where the mean sits relative to the centre of the tolerance — it only measures the width of the distribution against the width of the tolerance. Cpk takes centering into account: Cpk = min(Cpu, Cpl) → if the process is off-centre, Cpk is always lower than Cp. The gap Cp − Cpk is therefore a measure of off-centring. If Cp = 1.5 but Cpk = 0.8, the process has the capability but is shifted away from the target.
What Cpk does VW, Stellantis, BMW, Ford require?
For PPAP submission most OEMs accept the IATF baseline of 1.33 for normal characteristics. For safety / critical characteristics (CC, SC, S/C marked in the Control Plan) the typical requirement is 1.67. Some OEMs (Ford for Q1 supplier programmes, Daimler Truck for safety-relevant parts) require a minimum of 2.0 for safety. Always verify in customer-specific requirements (VW Formel Q, Stellantis SQAM, Ford Q1, BMW GS-Normen).
What should I do if Cpk is too low?
First verify the measurement itself — Gauge R&R (MSA) must be acceptable before judging the process. If measurement is OK: (1) if the process is off-centre (Cp ≫ Cpk), adjust the process setting. (2) If variability is too large (Cp low), root-cause via Ishikawa + 5 Why, look for the dominant variance source. (3) Containment if non-conforming parts may have shipped — 100% sort or stop production.
Does this calculator work offline?
After the page loads once, yes — the computation is pure JavaScript in the browser, no data goes to a server. You only need the tab open. For full offline use, install the QualityOS PWA (Add to Home Screen on mobile Chrome).

Built by an automotive quality engineer

This calculator exists because I opened it in Excel every single week.

12 years in automotive quality: TRW Automotive Torino (PFMEA expert), HELLA QPC (Nissan Navara + Mercedes-Benz launches), today Production Quality Leader at HELLA-Forvia Kočovce (24-person QA team across 2 factories, Tier 1 supplier to Tesla / Jaguar / Scania / Renault / Mercedes-Benz). VDA 6.3 Internal Auditor (Module A + B2), AIAG-VDA PFMEA moderator.

My Cpk file in Excel was always lost between 30 other spreadsheets. This is the version I keep open in a browser tab — works on mobile and desktop, no add-in, no IT involved.

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The calculator gave you Cpk. The AI tells you what to do with it.

The QualityOS AI Assistant ties your result to the specific characteristic in your Control Plan / PFMEA, suggests containment if Cpk dropped, and — in case of a customer complaint — drafts the customer email in the language your customer expects (VW DE, Stellantis IT, JLR EN).

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