Everything you need to know about AQL sampling inspection: how AQL tables work, which inspection levels to choose, how to read accept/reject numbers, and how to apply ISO 2859-1 to protect your supply chain. Whether you are an importer, quality manager, or sourcing professional, this guide will help you master AQL-based quality control.
AQL — Acceptable Quality Level (also called Acceptable Quality Limit) — is the maximum percentage of defective units in a production lot that is considered acceptable during a random sampling inspection. It is the cornerstone of statistical quality control for manufactured goods and is defined by the international standard ISO 2859-1 (also known as ANSI/ASQ Z1.4 in North America).
In practical terms, AQL answers a simple question: "How many defects are too many?" Rather than inspecting every single unit in an order — which is often impractical or prohibitively expensive — AQL sampling lets you draw a statistically representative sample from the lot, inspect those units, and make an accept-or-reject decision about the entire shipment based on the number of defects found.
AQL is expressed as a percentage. For example, an AQL of 2.5 means you are willing to accept a lot where up to 2.5% of units might be defective. The lower the AQL, the stricter the quality requirement. Typical AQL values range from 0.065% (extremely strict, used for critical safety defects) to 6.5% (lenient, used for cosmetic-only issues).
You can calculate the exact sample size and accept/reject numbers for any lot size using our free AQL calculator.
Statistical sampling inspection dates back to the 1920s, when Harold F. Dodge and Harry G. Romig at Bell Telephone Laboratories developed the first acceptance sampling tables. Their work laid the foundation for modern quality control by proving that inspecting a carefully chosen random sample could provide reliable information about an entire production lot.
During World War II, the U.S. military needed a standardised method to inspect the vast quantities of weapons, ammunition, and supplies being manufactured by thousands of contractors. The result was MIL-STD-105, first published in 1950 and updated several times through MIL-STD-105E in 1989. This military standard became the most widely used acceptance sampling plan in the world.
In 1999, the International Organization for Standardization adopted essentially the same methodology as ISO 2859-1: Sampling procedures for inspection by attributes — Part 1: Sampling schemes indexed by acceptance quality limit (AQL) for lot-by-lot inspection. The American version is published as ANSI/ASQ Z1.4. Despite the different names, all three standards use the same tables and procedures.
Today, ISO 2859-1 is the global standard used by importers, retailers, and quality control companies to determine how many units to inspect and how many defects are acceptable. Virtually every third-party inspection company — including Tetra Inspection — bases its sampling plans on this standard.
AQL-based inspection involves three core concepts that work together: inspection levels, sample sizes, and accept/reject numbers. Understanding all three is essential for using AQL correctly.
ISO 2859-1 defines three General Inspection Levels and four Special Inspection Levels. The inspection level determines how large a sample you will draw relative to the lot size:
The four Special Inspection Levels (S-1 through S-4) use much smaller sample sizes and are intended for situations where testing is destructive (e.g., tensile strength tests) or extremely expensive. They provide less statistical confidence but are practical when each unit tested is consumed or destroyed.
The sample size is determined by two factors: your lot size (the total number of units in the shipment) and your chosen inspection level. You first look up the sample size code letter (A through R) in Table A below, then use that code letter to find the actual number of units to inspect in Table B.
A common misconception is that larger lots always require proportionally larger samples. In reality, the sample size increases much more slowly than the lot size. For example, a lot of 5,000 units at Level II requires inspecting 200 units (4%), while a lot of 100,000 units at Level II requires inspecting 500 units (only 0.5%). This is a fundamental property of statistical sampling: beyond a certain sample size, additional units add diminishing returns in terms of detection power.
For each combination of sample size code letter and AQL value, the standard provides two numbers: the Accept number (Ac) and the Reject number (Re). After inspecting all units in the sample, you count the total number of defective units found.
If the number of defects found is equal to or less than the Accept number, the lot passes and is accepted. If the number of defects is equal to or greater than the Reject number, the lot fails and should be rejected or reworked. The Reject number is always exactly one more than the Accept number (Re = Ac + 1).
Below are the two key AQL tables from ISO 2859-1 that every quality professional should know. Table A converts your lot size and inspection level into a sample size code letter. Table B converts that code letter and your chosen AQL value into the actual sample size and accept/reject numbers.
| Lot Size | Special Levels | General Levels | |||||
|---|---|---|---|---|---|---|---|
| S-1 | S-2 | S-3 | S-4 | I | II | 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+ | D | E | H | K | N | Q | R |
Source: ISO 2859-1 / ANSI/ASQ Z1.4. General Inspection Level II (highlighted in cyan) is the default for most consumer goods.
| Code | Sample Size | 0.065 | 0.10 | 0.15 | 0.25 | 0.40 | 0.65 | 1.0 | 1.5 | 2.5 | 4.0 | 6.5 |
|---|---|---|---|---|---|---|---|---|---|---|---|---|
| Ac Re | Ac Re | Ac Re | Ac Re | Ac Re | Ac Re | Ac Re | Ac Re | Ac Re | Ac Re | Ac Re | ||
| A | 2 | — | — | — | — | — | — | — | — | 0 1 | 0 1 | 1 2 |
| B | 3 | — | — | — | — | — | — | — | 0 1 | 0 1 | 0 1 | 1 2 |
| C | 5 | — | — | — | — | — | — | 0 1 | 0 1 | 0 1 | 1 2 | 2 3 |
| D | 8 | — | — | — | — | — | 0 1 | 0 1 | 0 1 | 1 2 | 2 3 | 3 4 |
| E | 13 | — | — | — | — | 0 1 | 0 1 | 0 1 | 1 2 | 2 3 | 3 4 | 5 6 |
| F | 20 | — | — | — | 0 1 | 0 1 | 0 1 | 1 2 | 2 3 | 3 4 | 5 6 | 7 8 |
| G | 32 | — | — | 0 1 | 0 1 | 0 1 | 1 2 | 2 3 | 3 4 | 5 6 | 7 8 | 10 11 |
| H | 50 | — | 0 1 | 0 1 | 0 1 | 1 2 | 2 3 | 3 4 | 5 6 | 7 8 | 10 11 | 14 15 |
| J | 80 | 0 1 | 0 1 | 0 1 | 1 2 | 2 3 | 3 4 | 5 6 | 7 8 | 10 11 | 14 15 | 21 22 |
| K | 125 | 0 1 | 0 1 | 1 2 | 2 3 | 3 4 | 5 6 | 7 8 | 10 11 | 14 15 | 21 22 | — |
| L | 200 | 0 1 | 1 2 | 2 3 | 3 4 | 5 6 | 7 8 | 10 11 | 14 15 | 21 22 | — | — |
| M | 315 | 1 2 | 2 3 | 3 4 | 5 6 | 7 8 | 10 11 | 14 15 | 21 22 | — | — | — |
| N | 500 | 2 3 | 3 4 | 5 6 | 7 8 | 10 11 | 14 15 | 21 22 | — | — | — | — |
| P | 800 | 3 4 | 5 6 | 7 8 | 10 11 | 14 15 | 21 22 | — | — | — | — | — |
| Q | 1250 | 5 6 | 7 8 | 10 11 | 14 15 | 21 22 | — | — | — | — | — | — |
| R | 2000 | 7 8 | 10 11 | 14 15 | 21 22 | — | — | — | — | — | — | — |
Ac = Accept number, Re = Reject number. "—" means the plan is not available for that combination; use the next larger or smaller sample size. AQL 2.5 column (highlighted) is the most commonly used value for major defects.
For a detailed visual walkthrough of these tables, see our guide: AQL Chart Explained
You have a lot of 4,000 garments and want to inspect at General Inspection Level II with AQL 2.5 for major defects.
Step 1: In Table A, find the row for lot size 3,201–10,000. Under Level II, the code letter is L.
Step 2: In Table B, find row L. The sample size is 200 units. Under AQL 2.5, the accept/reject numbers are Ac = 21, Re = 22.
Result: Inspect 200 randomly selected garments. If you find 21 or fewer defective units, the lot passes. If you find 22 or more, the lot fails.
While the ISO 2859-1 standard defines AQL values from 0.065 to 6.5, most importers and quality control professionals work with a handful of commonly used levels. The choice depends on the type of defect and the product category.
Used for defects that pose a safety hazard or violate mandatory regulations. Examples include sharp edges on children's toys, electrical shock risks, toxic materials, or choking hazards. Many buyers set AQL 0.0 for critical defects (zero tolerance), meaning any critical defect found causes the lot to fail. ISO 2859-1 technically supports AQL values as low as 0.065 for these situations.
Applied to functional defects that significantly affect the product's usability but are not safety-related. Examples include electronic devices that fail to power on, garments with wrong sizing, or machinery with performance below specifications. AQL 1.0 is common in electronics, automotive parts, and medical devices.
A stricter threshold for major defects, used by retailers and brands with high quality standards. Suitable for premium consumer goods, branded apparel, and products where returns are costly. AQL 1.5 gives a tighter acceptance criterion than the standard AQL 2.5 while remaining practical for large production lots.
The most widely used AQL value in international trade. AQL 2.5 is the industry default for major defects across virtually all consumer product categories — from garments and footwear to electronics, homeware, and promotional items. If you are unsure which AQL to use for major defects, AQL 2.5 is nearly always the correct starting point.
Used for cosmetic imperfections that do not affect the product's function, safety, or salability. Examples include minor colour variations, tiny scratches not visible at arm's length, or slightly uneven stitching. AQL 4.0 is the standard for minor defects in most inspection protocols. Some buyers use AQL 6.5 for very low-impact cosmetic issues.
Choosing the right AQL is not a one-size-fits-all decision. It depends on your product's risk profile, your target market's expectations, your relationship with the supplier, and the specific defect categories you are tracking. Here is a practical framework:
For new suppliers or first orders: use stricter AQL levels (e.g., AQL 1.5 for major, AQL 2.5 for minor). A new supplier has not yet proven their quality consistency, so tighter acceptance criteria reduce your risk. As the supplier demonstrates reliable quality over multiple shipments, you can gradually relax to standard levels.
For high-value or safety-critical products: always use AQL 0.0 for critical defects and AQL 1.0 or lower for major defects. Products sold to children, used in medical or food applications, or carrying regulatory certifications (CE, UL, CPSC) require the strictest standards. The cost of a recall or liability claim far outweighs the cost of tighter inspection.
Tetra Inspection can help you define the right AQL levels for your specific products and markets. Learn more about our pre-shipment inspection services.
For practical advice on setting AQL levels with suppliers, read: AQL Inspection Best Practices for Importers
Different product categories have different risk profiles, defect patterns, and industry norms. Here is how AQL is typically applied across major product categories:
Standard AQL: 2.5 major / 4.0 minor. Key defects include sizing errors, fabric flaws, stitching issues, and colour deviations. Measure at least 5 garments per size for measurement checks. Premium brands often use AQL 1.5 for major defects.
Standard AQL: 1.0–2.5 major / 4.0 minor / 0.0 critical. Functional testing is essential — every sampled unit should be powered on and tested. Key defects include power failure, screen defects, connectivity issues, and safety risks (overheating, sharp internals).
Standard AQL: 0.0 critical / 1.5 major / 4.0 minor. Safety is paramount. All sampled units must pass small parts testing (ASTM F963, EN 71), sharp edge testing, and age-appropriate labelling verification. Regulatory compliance defects are always critical.
Standard AQL: 0.0 critical / 1.5 major / 4.0 minor. All materials must comply with FDA 21 CFR, EU Regulation 1935/2004, or equivalent local standards. Key defects include contamination, incorrect labelling, missing certifications, and material composition failures.
Standard AQL: 2.5 major / 4.0 minor. Key defects include structural weakness, assembly issues, surface finish flaws, and dimension errors. Weight-bearing and stability tests should be included for all seating and storage furniture.
Standard AQL: 0.65–1.0 critical / 2.5 major / 4.0 minor. Product integrity and safety are critical. Key defects include contamination, incorrect formulation, packaging leaks, and labelling non-compliance. Batch testing and certificate verification are essential.
AQL and sampling plans are two sides of the same coin. The AQL value defines your quality threshold — the maximum percentage of defects you will tolerate. The sampling plan is the operational procedure — how many units to sample and the accept/reject decision rule — that enforces that threshold with statistical confidence.
ISO 2859-1 provides three types of sampling plans: single sampling (inspect one sample, make a decision), double sampling (inspect a first sample, and if results are borderline, inspect a second sample before deciding), and multiple sampling (up to seven successive samples). Single sampling is by far the most common in third-party inspection because it is simpler to execute and audit.
ISO 2859-1 includes a built-in mechanism for adjusting inspection severity based on the supplier's recent quality performance. These switching rules reward good suppliers with less inspection and penalise poor suppliers with more:
These switching rules create a powerful incentive structure: suppliers who consistently deliver good quality are rewarded with faster, cheaper inspections, while those who deliver marginal quality face increasingly strict scrutiny.
Count the total number of finished units in the production lot or shipment. This is the total quantity produced and ready for inspection, not the quantity you plan to inspect. If production is split across multiple shipments, each shipment is treated as a separate lot.
Select the appropriate inspection level. For most consumer products, use General Inspection Level II (the industry default). Choose Level I for lower-risk items or destructive testing, and Level III for high-risk or safety-critical products. Special Levels (S-1 to S-4) are used only when testing is destructive or prohibitively expensive.
In Table A, find the row that matches your lot size range. Read across to your chosen inspection level column to find the code letter (A through R). This letter links your lot size and inspection level to a specific sample size in Table B.
Select an AQL percentage for each defect category you are tracking. Most inspections use three: AQL 0.0 for critical defects (zero tolerance), AQL 2.5 for major defects, and AQL 4.0 for minor defects. Adjust based on your product type and risk tolerance.
In Table B, find the row for your code letter. The second column shows your sample size — the number of units to randomly select and inspect. Then read across to your AQL column to find the Accept (Ac) and Reject (Re) numbers. If you find Ac or fewer defects, the lot passes. If you find Re or more defects, the lot fails.
Lot: 8,000 units | Level: General II | AQL: 2.5 (major), 4.0 (minor)
Table A → lot size 3,201–10,000 at Level II → code letter L
Table B → code L → sample size 200
AQL 2.5 → Ac 21, Re 22 (major defects)
AQL 4.0 → use code L row, but AQL 4.0 shows "—" → step up to next available: Ac 21, Re 22 at code K (125 units).
Inspect 200 units. Count major and minor defects separately. Compare each count to its accept/reject pair.
Want to calculate your sample size automatically? Try our AQL Calculator Guide
One of the most common questions importers ask is: "Why not just inspect every single unit?" The answer involves cost, time, human factors, and — perhaps surprisingly — inspection accuracy.
100% inspection means examining every unit in the lot. While this sounds thorough, it has significant practical limitations. Inspection fatigue is a well-documented phenomenon: after inspecting hundreds of identical units, even experienced inspectors lose concentration and start missing defects. Studies show that 100% visual inspection typically catches only 80–85% of defects due to human fatigue. AQL-based sampling, where inspectors examine fewer units but with higher concentration, can actually detect defects more reliably per unit inspected.
For the vast majority of consumer goods imported from overseas factories, AQL-based sampling inspection is the industry standard and provides the best balance of cost, speed, and statistical confidence. Reserve 100% inspection for special situations where the risk or value justifies the additional expense.
AQL stands for Acceptable Quality Level (also called Acceptable Quality Limit). It is the maximum percentage of defective units in a production lot that a buyer is willing to accept. AQL is defined by the international standard ISO 2859-1 and the equivalent American standard ANSI/ASQ Z1.4.
AQL 2.5 is the most commonly used value worldwide for major defects in consumer goods. It is the industry default used by most importers, retailers, and third-party inspection companies. For minor defects, AQL 4.0 is standard, and for critical defects, AQL 0.0 (zero tolerance) is typical.
Lot size is the total number of units in the production batch being inspected. AQL is the acceptable defect rate expressed as a percentage. Together, the lot size and AQL (along with the inspection level) determine how many units to sample and how many defects are allowed for the lot to pass.
Yes. AQL 0.0 means zero defects are accepted in the sample — if even one defective unit is found, the lot fails. This is standard practice for critical defects (safety hazards, regulatory violations). Note that AQL 0.0 does not guarantee zero defects in the entire lot; it only means zero were found in the sample.
AQL (Acceptable Quality Limit) is the quality level you consider acceptable — lots at this quality level have a high probability of being accepted. LTPD (Lot Tolerance Percent Defective) is the quality level you consider unacceptable — lots at this level have a high probability of being rejected. Together, AQL and LTPD define the operating characteristics of a sampling plan.
AQL-based sampling is the standard for final random inspection (pre-shipment inspection). However, not all quality control activities use AQL. During-production inspections may use AQL on the finished portion. Factory audits, lab testing, and initial production checks use different methodologies. AQL is most useful when you have a completed lot of identical products ready for shipment.
If a lot fails (defects found ≥ Reject number), the buyer typically has several options: reject the entire lot, require the factory to sort and rework all units (100% sorting), negotiate a price reduction, or request a re-inspection after the factory has addressed the defects. The best approach depends on the nature and severity of the defects.
The AQL 2.5 standard is defined by ISO 2859-1 and the sampling tables are universal. However, different industries may apply different AQL values for their specific defect categories. For example, the automotive industry often uses AQL 0.65 for functional defects, while the garment industry commonly uses AQL 2.5. The tables are the same; the chosen AQL values differ.
AQL and Six Sigma are complementary but different approaches. AQL is an acceptance sampling method — it determines whether to accept or reject a finished lot. Six Sigma is a process improvement methodology focused on reducing variation and defects during manufacturing. A factory running at Six Sigma (3.4 defects per million) would easily pass any AQL inspection. AQL is about inspection; Six Sigma is about prevention.
Yes, and you should. The standard practice is to set separate AQL values for critical, major, and minor defects. A typical configuration is AQL 0.0 for critical defects, AQL 2.5 for major defects, and AQL 4.0 for minor defects. Each defect category is evaluated independently — a lot could pass for minor defects but fail for major defects.
Dive deeper into AQL topics with these in-depth articles from our quality control blog.
Use our free AQL calculator to determine your sample size instantly, or let Tetra Inspection handle the entire inspection process for you — from sampling plan to detailed report.
