This cheat sheet is the fast reference for writing and reading AQL-based inspection specs. It summarizes what inspection levels change (sample size), how AQL maps to Critical/Major/Minor defect severity, and includes a few worked examples that show why "AQL 2.5" is not "2.5% defects." Use it to sanity-check inspection reports, align expectations with suppliers and inspectors, and avoid arguments caused by vague defect classification.
AQL Calculator
What this calculator does This calculator helps you interpret an inspection report that uses AQL sampling. It estimates: * The sample size (n) from lot size and inspection level * The accept/reject thresholds (Ac/Re) for Critical, Major, and Minor defects Then you can check whether the reported defect counts should
Yury Galtykhin
Inspection Levels (Quick Reference)
Inspection level mainly changes the sample size (cost vs discrimination). Level II is the default in most programs.
General Level I
- Changes: Smaller sample vs Level II
- Use: Low-risk / low-cost checks
- Tradeoff: Cheaper, higher sampling risk
General Level II (default)
- Changes: Baseline sample size
- Use: Most routine inspections
- Tradeoff: Best balance of cost vs risk
General Level III
- Changes: Larger sample vs Level II
- Use: Higher-risk / higher-value lots
- Tradeoff: More discrimination, more cost/time
Special S-1 to S-4
- Changes: Very small samples (S-1 smallest → S-4 largest)
- Use: Destructive tests / limited sampling
- Tradeoff: Highest sampling risk tolerated
AQL by Defect Severity (Baseline)
These are common consumer-goods defaults. Tighten based on risk, brand promises, and return economics.
Critical
- Meaning: Safety / regulatory / cannot use
- Baseline: AQL 0 (Ac 0 / Re 1)
- Notes: Zero acceptance is typical
Major
- Meaning: Function / performance / usability
- Baseline: AQL 2.5
- Tighten: 1.5 / 1.0 for new suppliers, early runs
Minor
- Meaning: Cosmetic; does not affect use
- Baseline: AQL 4.0
- Tighten: Premium finishes / high-visibility surfaces
Typical spec line: General Level II: Critical AQL 0, Major AQL 2.5, Minor AQL 4.0
Worked Examples (AQL 2.5 is not “2.5% defects”)
Passing means defects found ≤ Ac in the sample — not that the lot defect rate equals AQL.
n = 80 (Major AQL 2.5)
- Pass/Fail: Ac 5 / Re 6
- Sample math: 5/80 = 6.25%
n = 125 (Major AQL 2.5)
- Pass/Fail: Ac 7 / Re 8
- Sample math: 7/125 = 5.6%
n = 200 (Major AQL 2.5)
- Pass/Fail: Ac 10 / Re 11
- Sample math: 10/200 = 5.0%
n = 315 (Major AQL 2.5)
- Pass/Fail: Ac 14 / Re 15
- Sample math: 14/315 = 4.44%
Note: Full “lot size → sample size → Ac/Re” sampling plans are in ISO 2859-1 / ANSI Z1.4.