AQL Levels Explained: 2.5, Sampling, and Worked Examples

AQL stands for Acceptable Quality Limit. The number is expressed as a percentage of defective pieces. An AQL level of 2.5 therefore sets a decision threshold at 2.5% defects in the inspected sample. This threshold answers a simple question: should the lot be accepted or rejected after inspection?

The rule comes from the ISO 2859-1 standard, dedicated to sampling for attribute inspections. The American equivalent is the ANSI/ASQ Z1.4 standard. In China, factories often work with GB/T 2828.1, the national equivalent, which uses the same calculation principle. Our sinophone team in Guangzhou relies on these references to set the thresholds with a factory, in the local language, without room for interpretation.

Three parameters determine the sample: the lot size, the inspection level, and the selected AQL. The general inspection level II is the default provided by the standard. Levels I and III also exist: the first lightens the inspection, the second tightens it. Depending on the selected level, the code letter table ranges from A to R: the further the letter, the larger the sample.

In practice, you cross the lot size with the inspection level: this gives a code letter. This letter corresponds to a sample size. Crossing this size with the AQL yields two numbers, acceptance and rejection, noted Ac and Re. If the sample has at most Ac defects, the lot passes. At Re, it is rejected.

Take a lot of 3,200 pieces inspected at general inspection level II. The table gives letter K and a sample of 125 pieces. With an AQL of 1.0, the lot is accepted up to 3 defects and rejected from the 4th. With an AQL of 2.5, it is accepted up to 7 defects and rejected from the 8th. With an AQL of 4.0, it passes up to 10 defects and fails from the 11th.

Compare with a small lot of 150 pieces. Letter F requires a sample of 20 pieces. At AQL 2.5, we accept up to 1 defect and reject from the 2nd. The lot is inspected on fewer pieces, but the margin tightens: each counted defect weighs more. In both cases, we only look at a fraction of the quantity: about 4% of the large lot, about 13% of the small.

ISO 2859-1 does not define any defect classes. The distinction between critical, major, and minor defects is a convention of the quality control industry. The trio 0 · 2.5 · 4.0 has become standard in a large part of sourcing: zero tolerance for critical, 2.5 for major, 4.0 for minor. These values remain a starting point, not a regulatory obligation.

A critical defect affects safety or regulations: sharp edge, poorly insulated cable, missing mandatory marking. A single unit is enough to reject the lot. A major defect makes the product hard to sell or non-compliant with specifications: open seam, dimension out of tolerance, malfunctioning feature. A minor defect relates to finishing: small snag, print misalignment, color shade. These definitions must be written, with concrete examples, before inspection.

An accepted lot is not a perfect lot. Let's take again the lot of 3,200 pieces at AQL 2.5: a lot that contains about 2% defective pieces, i.e., about sixty units, has a good chance of passing. The AQL is therefore neither a promise of quality nor a cap on defects. It is a balance point between two risks: that of the supplier, who sees a good lot rejected by sampling bad luck, and that of the buyer, who sees a bad lot accepted.

The standard provides switching rules: if two out of five consecutive lots are rejected at the first inspection, the inspection switches to tightened mode, with more demanding thresholds. Another limit, less technical: the result depends on the precision of the defect definitions and the inspector's eye. Two inspections of the same lot can give different conclusions if the specifications remain vague.

Levels are not chosen on the day of inspection. They depend on the product's use, regulatory requirements of your market, sales channel, and customer sensitivity. A toy intended for toddlers is not treated like a decorative item: the critical class absorbs defects that an adult buyer would not notice.

Write your AQL thresholds and defect definitions in the specification sheet, proforma, or purchase contract. Mention the standard and inspection level: ISO 2859-1, general level II. Send the same document to the factory and the inspector. This document avoids post-hoc discussions when the goods are already produced and time is short.

Our brokerage and sourcing firm works with a Chinese-speaking team based in Guangzhou. This team defines AQL thresholds and defect definitions with the factory, in the local language, before production starts. We then arrange pre-shipment inspection and review each report with you: number of defects, breakdown by class, acceptance or rejection decision.

In most cases, you pay no fees: you open a file, we negotiate the goods for you, and we take a commission on their ex-factory value.