What Is AQL (Acceptable Quality Level)?
AQL stands for Acceptable Quality Level. It is a statistical measure defined in the international standard ISO 2859-1 (also published as ANSI/ASQ Z1.4) that specifies the maximum percentage of defective units in a sample for the production lot to be considered acceptable.
AQL is the foundation of modern quality control inspection. Rather than checking every single unit in a production run — which would be prohibitively expensive and time-consuming — inspectors draw a random sample from the lot and evaluate it. The AQL system determines how many units to sample and how many defective units can be found before the entire lot is rejected.
The Three AQL Defect Categories
| Defect Type | Typical AQL | Definition | Example |
|---|---|---|---|
| Critical | 0 (zero) | Poses a safety hazard or violates regulations. No defective units are acceptable. | Sharp edges on a toy, electrical shock risk, toxic materials |
| Major | 2.5 | Renders the product unusable, unsaleable, or significantly below spec. A consumer would reject it. | Broken zipper, non-functional button, wrong color, missing component |
| Minor | 4.0 | A cosmetic or slight deviation from spec that does not affect function or salability. | Small scratch, loose thread, slight color variation, minor stain |
How AQL Sampling Works
The AQL inspection process follows three steps: determine lot size, find the sample size code letter, then look up sample size and accept/reject numbers.
Determine Lot Size
Count the total number of finished units in the production lot ready for inspection.
Find Code Letter
Use Table 1 below with your lot size and inspection level to get the sample size code letter (A through R).
Look Up Sample Size
Use Table 2 with the code letter and your AQL value to find the sample size and accept/reject numbers.
Inspect & Decide
Draw a random sample, count defects. If defects ≤ Ac (accept number), the lot passes. If defects ≥ Re (reject number), the lot fails.
Table 1: Sample Size Code Letters (ISO 2859-1)
This table maps your lot size and inspection level to a code letter. General Inspection Level II is the standard used in the vast majority of quality inspections worldwide.
| Lot Size | Special S-1 | Special S-2 | Special S-3 | Special S-4 | Level I | Level II | Level III |
|---|---|---|---|---|---|---|---|
| 2 – 8 | A | A | A | A | A | A | B |
| 9 – 15 | A | A | A | A | A | B | C |
| 16 – 25 | A | A | B | B | B | C | D |
| 26 – 50 | A | B | B | C | C | D | E |
| 51 – 90 | B | B | C | C | C | E | F |
| 91 – 150 | B | B | C | D | D | F | G |
| 151 – 280 | B | C | D | E | E | G | H |
| 281 – 500 | B | C | D | E | F | H | J |
| 501 – 1,200 | C | C | E | F | G | J | K |
| 1,201 – 3,200 | C | D | E | G | H | K | L |
| 3,201 – 10,000 | C | D | F | G | J | L | M |
| 10,001 – 35,000 | C | D | F | H | K | M | N |
| 35,001 – 150,000 | D | E | G | J | L | N | P |
| 150,001 – 500,000 | D | E | G | J | M | P | Q |
| 500,001 and over | D | E | H | K | N | Q | R |
Table 2: Single Sampling Plans — General Inspection Level II
Once you have the code letter from Table 1, use this table to find the sample size and the accept (Ac) / reject (Re) numbers for common AQL values. This is the most-referenced AQL table in quality control.
| Code Letter | Sample Size | AQL 0.65 | AQL 1.0 | AQL 1.5 | AQL 2.5 | AQL 4.0 | AQL 6.5 | ||||||
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| Ac | Re | Ac | Re | Ac | Re | Ac | Re | Ac | Re | Ac | Re | ||
| A | 2 | - | - | - | - | - | - | 0 | 1 | 0 | 1 | 0 | 1 |
| B | 3 | - | - | - | - | 0 | 1 | 0 | 1 | 0 | 1 | 0 | 1 |
| C | 5 | - | - | 0 | 1 | 0 | 1 | 0 | 1 | 0 | 1 | 1 | 2 |
| D | 8 | 0 | 1 | 0 | 1 | 0 | 1 | 0 | 1 | 1 | 2 | 1 | 2 |
| E | 13 | 0 | 1 | 0 | 1 | 0 | 1 | 1 | 2 | 1 | 2 | 2 | 3 |
| F | 20 | 0 | 1 | 0 | 1 | 1 | 2 | 1 | 2 | 2 | 3 | 3 | 4 |
| G | 32 | 0 | 1 | 1 | 2 | 1 | 2 | 2 | 3 | 3 | 4 | 5 | 6 |
| H | 50 | 1 | 2 | 1 | 2 | 2 | 3 | 3 | 4 | 5 | 6 | 7 | 8 |
| J | 80 | 1 | 2 | 2 | 3 | 3 | 4 | 5 | 6 | 7 | 8 | 10 | 11 |
| K | 125 | 2 | 3 | 3 | 4 | 5 | 6 | 7 | 8 | 10 | 11 | 14 | 15 |
| L | 200 | 3 | 4 | 5 | 6 | 7 | 8 | 10 | 11 | 14 | 15 | 21 | 22 |
| M | 315 | 5 | 6 | 7 | 8 | 10 | 11 | 14 | 15 | 21 | 22 | 21 | 22 |
| N | 500 | 7 | 8 | 10 | 11 | 14 | 15 | 21 | 22 | 21 | 22 | 21 | 22 |
| P | 800 | 10 | 11 | 14 | 15 | 21 | 22 | 21 | 22 | 21 | 22 | 21 | 22 |
| Q | 1250 | 14 | 15 | 21 | 22 | 21 | 22 | 21 | 22 | 21 | 22 | 21 | 22 |
| R | 2000 | 21 | 22 | 21 | 22 | 21 | 22 | 21 | 22 | 21 | 22 | 21 | 22 |
How to Read an AQL Table: Step-by-Step Example
Let us walk through a real example to demonstrate how the AQL calculator works in practice.
Scenario: 5,000 units of garments, standard inspection
- Lot size: 5,000 units
- Inspection level: General Inspection Level II (standard)
- Go to Table 1: Lot size 3,201 – 10,000 at Level II = Code Letter L
- Go to Table 2: Code Letter L = Sample size 200 units
- For AQL 2.5 (major defects): Ac = 10, Re = 11
- For AQL 4.0 (minor defects): Ac = 14, Re = 15
AQL Calculator
Use this calculator to quickly find your sample size and accept/reject numbers. For the full interactive experience with custom AQL values and multiple inspection levels, use our advanced online AQL calculator.
Quick AQL Sample Size Calculator
AQL Sampling Plan
Need a more advanced AQL inspection calculator with custom parameters, switching plans, and exportable reports? Use the full AQM BD calculator →
AQL Inspection Levels Explained
ISO 2859-1 defines seven inspection levels — three general and four special. The inspection level determines the relationship between the lot size and the sample size. A higher level means a larger sample and better defect detection, but also higher inspection costs.
General Inspection Levels (I, II, III)
| Level | Sample Size | When to Use |
|---|---|---|
| Level I (Reduced) | Smaller than Level II | Established supplier with consistently good quality. Lower inspection cost but less protection against defects. |
| Level II (Normal) | Standard | The default for most inspections. Used unless there is a specific reason to choose another level. Recommended for new suppliers and general trade inspections. |
| Level III (Tightened) | Larger than Level II | When higher confidence is needed. Used after a supplier has failed previous inspections or for high-risk/high-value products. |
Special Inspection Levels (S-1 to S-4)
Special levels are designed for situations where the sample size must be kept very small — typically because the test is destructive, expensive, or time-consuming.
- S-1: Smallest sample. Used for very expensive or destructive tests (e.g., burst-pressure testing).
- S-2: Slightly larger. Suitable for chemical analysis or tensile strength tests.
- S-3: Moderate special sample. Used when some destructive testing is needed but more data points help.
- S-4: Largest special level. Approaches Level I general in sample size. Used for semi-destructive tests like wash tests on garments.
Common AQL Levels by Industry
While the standard AQL values of 0/2.5/4.0 are widely used, different industries may apply stricter or more relaxed criteria depending on the product risk profile and buyer requirements.
Garments & Textiles
Most common in apparel QC. Standard levels apply for mass-market products.
Electronics & Electrical
Tighter tolerances due to safety and functionality concerns.
Packaging & Printing
Focus on visual consistency and print quality for brand image.
Food & Beverages
Strict criteria due to health and safety regulations. Often zero tolerance for contamination.
Toys & Children's Products
Heavily regulated. Safety defects (choking hazards, sharp edges) have zero tolerance.
Furniture & Homeware
Structural integrity is key. Surface finish defects may be more tolerated on hidden surfaces.
Common Mistakes When Using AQL
Even experienced quality professionals sometimes misapply AQL sampling. Here are the most frequent errors to avoid:
1. Using the Wrong Lot Size
The lot size is the total number of finished units available at the time of inspection — not the purchase order quantity. If your order is 10,000 units but only 6,000 are packed when the inspector arrives, the lot size is 6,000.
2. Not Randomizing the Sample
AQL requires a truly random sample drawn from all cartons in the lot. Checking only the cartons nearest to the door or the ones the factory presents first introduces bias and undermines the statistical validity of the inspection.
3. Confusing AQL with Defect Rate
An AQL of 2.5 does not mean you are accepting 2.5% defective products. It means that if the true process defect rate is at or below 2.5%, the lot has a high probability (approximately 95%) of being accepted. If the actual defect rate is higher, the probability of rejection increases sharply.
4. Using Special Levels to Cut Costs
Some buyers request Special Level S-1 or S-2 for the entire inspection to reduce sample sizes and costs. This dramatically reduces defect detection ability. Special levels are designed only for destructive or expensive tests, not general visual inspection.
5. Ignoring the Switching Rules
ISO 2859-1 includes switching rules: after a series of accepted lots, you can switch from normal to reduced inspection. After rejections, you must switch to tightened inspection. Ignoring these rules means you may be over- or under-inspecting relative to the supplier's actual quality performance.
6. Not Defining Defect Classifications
Before the inspection begins, the buyer must provide a clear defect classification list that defines which defects are critical, major, and minor. Without this, the inspector must make subjective judgments, leading to inconsistent results.