AQL Definition
The Acceptable Quality Level (AQL) — also called the Acceptance Quality Limit — is the worst tolerable process average when a continuing series of lots is submitted for acceptance sampling. In simpler terms, it is the maximum percentage of defective items that can be found in a batch before the entire lot is rejected.
AQL is defined in ISO 2859-1 (also known as ANSI/ASQ Z1.4 in the United States and NF X06-022 in France). The standard provides statistical sampling plans that tell inspectors exactly how many items to check and how many defects to allow before rejecting a shipment.
Key point: AQL does not mean zero defects. It acknowledges that in mass production, some level of imperfection is statistically inevitable. AQL defines the threshold at which the defect rate is still considered commercially acceptable.
How AQL Works
AQL-based inspection follows a three-step process:
- Determine the lot size — Count the total number of units in the production batch.
- Find the sample size — Using ISO 2859-1 Table 1, match your lot size and inspection level to get a code letter. Then look up the code letter in Table 2 to find how many units to inspect.
- Apply accept/reject criteria — Inspect the sample. If the number of defective units is at or below the accept number (Ac), the lot passes. If it reaches the reject number (Re), the lot fails.
Example
A shipment of 5,000 units at General Inspection Level II gives code letter L and a sample size of 200 units. At AQL 2.5:
- Accept (Ac) = 10 — If 10 or fewer defects are found, the lot passes.
- Reject (Re) = 11 — If 11 or more defects are found, the lot is rejected.
Defect Classification
AQL inspection distinguishes three categories of defects, each with different tolerance levels:
| Defect Type | Typical AQL | Description | Examples |
|---|---|---|---|
| Critical | 0 – 0.65 | Safety hazards, regulatory violations, renders product unusable or dangerous | Sharp edges on toys, electrical shorts, toxic materials, missing safety labels |
| Major | 1.0 – 2.5 | Functional failures, significantly reduces usability or saleability | Broken zippers, wrong dimensions, non-functional buttons, color mismatch to spec |
| Minor | 2.5 – 4.0 | Cosmetic issues that don't affect function but deviate from specifications | Small scratches, loose threads, slight color variation, minor packaging damage |
Industry standard: The most common AQL combination used worldwide is 0 / 2.5 / 4.0 (Critical / Major / Minor). This means zero tolerance for critical defects, up to 2.5% major defects, and up to 4.0% minor defects.
Inspection Levels
ISO 2859-1 defines seven inspection levels that control the relationship between lot size and sample size:
| Level | Type | Sample Size | When to Use |
|---|---|---|---|
| S-1 | Special | Very small | Destructive testing (e.g., tensile strength, burst testing) |
| S-2 | Special | Small | Expensive or time-consuming tests |
| S-3 | Special | Small | Tests requiring specialized equipment |
| S-4 | Special | Moderate | When testing cost is a concern but more data is needed than S-1 to S-3 |
| GI (Level I) | General | Reduced | Trusted suppliers with proven quality history |
| GII (Level II) | General | Standard | Default for most inspections — best balance of cost and confidence |
| GIII (Level III) | General | Larger | High-risk products, new suppliers, or after a failed inspection |
General Inspection Level II is used in the vast majority of pre-shipment inspections worldwide. It provides the best trade-off between inspection cost and statistical confidence.
AQL Sample Size Table
This reference table shows sample sizes and accept/reject numbers for common lot sizes at General Inspection Level II:
| Lot Size | Code | Sample | AQL 1.0 Ac / Re |
AQL 2.5 Ac / Re |
AQL 4.0 Ac / Re |
|---|---|---|---|---|---|
| 2 – 8 | A | 2 | ↓ | ↓ | 0 / 1 |
| 9 – 15 | B | 3 | ↓ | ↓ | 0 / 1 |
| 16 – 25 | C | 5 | ↓ | 0 / 1 | 0 / 1 |
| 26 – 50 | D | 8 | ↓ | 0 / 1 | 1 / 2 |
| 51 – 90 | E | 13 | 0 / 1 | 1 / 2 | 1 / 2 |
| 91 – 150 | F | 20 | 0 / 1 | 1 / 2 | 2 / 3 |
| 151 – 280 | G | 32 | 1 / 2 | 2 / 3 | 3 / 4 |
| 281 – 500 | H | 50 | 1 / 2 | 3 / 4 | 5 / 6 |
| 501 – 1,200 | J | 80 | 2 / 3 | 5 / 6 | 7 / 8 |
| 1,201 – 3,200 | K | 125 | 3 / 4 | 7 / 8 | 10 / 11 |
| 3,201 – 10,000 | L | 200 | 5 / 6 | 10 / 11 | 14 / 15 |
| 10,001 – 35,000 | M | 315 | 7 / 8 | 14 / 15 | 21 / 22 |
| 35,001 – 150,000 | N | 500 | 10 / 11 | 21 / 22 | 21 / 22 |
| 150,001 – 500,000 | P | 800 | 14 / 15 | 21 / 22 | 21 / 22 |
↓ = Use next larger sample size. Based on ISO 2859-1 Table 2-A (Normal inspection, single sampling).
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Book Inspection →Industries That Use AQL
AQL-based sampling inspection is used across virtually every manufacturing sector:
- Consumer electronics — PCB assembly, components, finished devices
- Textiles & apparel — Garments, footwear, accessories
- Food & beverage — Packaging integrity, labeling, fill levels
- Pharmaceuticals — GMP compliance, packaging, labeling (often stricter AQL 0.065–0.65)
- Automotive — Components, sub-assemblies, interior trim
- Packaging — Corrugated boxes, labels, films, bags
- Toys & children's products — Safety-critical with AQL 0 for critical defects
- Medical devices — Often requires tighter AQL 0.10–0.65 for all defect types
Normal, Tightened, and Reduced Inspection
ISO 2859-1 includes switching rules that adjust inspection severity based on quality history:
Normal Inspection
The default starting point. Used when there is no evidence that the quality level is better or worse than the specified AQL.
Tightened Inspection
Triggered when 2 out of 5 consecutive lots are rejected under normal inspection. Uses the same sample size but stricter accept/reject criteria (lower accept numbers). Production must be halted if 5 consecutive lots are rejected under tightened inspection.
Reduced Inspection
Available when 10 consecutive lots are accepted under normal inspection and production is stable. Uses a smaller sample size, reducing inspection cost and time.
AQL vs. Other Quality Metrics
| Metric | Standard | Focus | Use Case |
|---|---|---|---|
| AQL | ISO 2859-1 | Attributes (pass/fail per unit) | Pre-shipment inspection, incoming goods |
| LTPD | ISO 2859-2 | Lot tolerance (consumer protection) | Isolated lots, one-time purchases |
| Cpk/Ppk | ISO 22514 | Process capability (variables data) | Manufacturing process control, SPC |
| Six Sigma | — | 3.4 DPMO target | Process improvement methodology |
| Zero Defect | — | 100% inspection | Aerospace, medical, safety-critical |
Frequently Asked Questions
AQL stands for Acceptable Quality Level (formerly Acceptable Quality Limit). It is defined in ISO 2859-1 as the quality level that is the worst tolerable process average when a continuing series of lots is submitted for acceptance sampling.
The most widely used AQL levels are 0.65 for critical defects, 2.5 for major defects, and 4.0 for minor defects. These values are standard across consumer goods, electronics, textiles, and most manufactured products.
AQL (Acceptable Quality Level) is the defect rate the buyer considers acceptable, while LTPD (Lot Tolerance Percent Defective) is the defect rate the buyer considers unacceptable. AQL protects the producer from good lots being rejected; LTPD protects the consumer from bad lots being accepted.
Sample size is determined by two factors: the lot size and the inspection level. Using ISO 2859-1 Table 1, find the code letter for your lot size and inspection level (usually General Inspection Level II). Then use Table 2-A (normal inspection) to find the sample size and accept/reject numbers for your chosen AQL value.
Defects are classified as: Critical (AQL 0 to 0.65) — safety hazards or regulatory violations; Major (AQL 1.0 to 2.5) — defects that significantly reduce usability; Minor (AQL 2.5 to 4.0) — cosmetic issues that don't affect function but deviate from specifications.
No. AQL explicitly acknowledges that zero defects is statistically impractical in mass production. Instead, AQL sets a practical threshold for the maximum acceptable defect rate. Industries requiring near-zero defects (aerospace, medical devices) typically use 100% inspection or tighter sampling plans rather than standard AQL.