AQL sampling explained: what AQL 2.5 really means

What is AQL?

In practice, an AQL plan tells the inspector how many units to pick at random from a lot, and how many defects of each class the sample may contain before the lot is rejected. It does not promise that any single lot contains fewer defects than the AQL.

For a one-paragraph definition, see the AQL glossary entry. To get the plan for your own lot, use the AQL calculator.

ISO 2859-1:2026, the standard behind AQL sampling

AQL sampling plans come from 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 current edition is ISO 2859-1:2026, the 3rd edition, published in January 2026. The ISO catalogue says it replaces the 1999 edition and its amendments, and adds skip-lot sampling procedures and updated guidance on applying sampling strategies. ANSI/ASQ Z1.4, published by ASQ, uses the same code letters and single sampling plans for the AQL columns shown on this page.

The tables below are the single sampling plans for normal inspection as long published in ISO 2859-1 and ANSI/ASQ Z1.4. We have not compared them line by line with the 2026 text, which ISO sells. Name the edition (the year) in your contract and inspection plan so the factory, the inspector and you all use the same tables.

How the tables work

1. Find the code letter. Look up the lot size and the inspection level in Table 1. The result is a sample size code letter.
2. Find the plan. In Table 2, the code letter gives the sample size, and the AQL column gives the accept number (Ac) and reject number (Re).
3. Inspect and compare. Inspect the sample. If the number of defects of a class is at or below Ac, that class passes; at or above Re, it fails. Each class (critical, major, minor) is judged on its own.

If the sample size is equal to or larger than the lot, inspect every unit.

Table 1: sample size code letters

Table 2: single sampling plans, normal inspection

Each cell shows Ac / Re. Where the printed standard shows an arrow instead of numbers, you use the first sampling plan in that column in the direction of the arrow (below for ↓, above for ↑), and its sample size. In the table below, arrow cells show the plan the arrow leads to and the sample size (n) that comes with it.

Example of the arrow rule: a lot of 50 at general level II is code D (8 samples). In the AQL 1.0 column, row D has an arrow down, so you use the first plan below it: row E, 13 samples, accept 0, reject 1. The sample size changes from 8 to 13.

This page covers normal inspection only. ISO 2859-1 also defines tightened and reduced inspection with switching rules for a continuing series of lots.

Inspection levels

Defect classes with furniture examples

Worked example: 800 dressers

An order of 800 solid-wood dressers, general inspection level II, AQL 0 / 2.5 / 4.0:

1. Table 1: 800 is in the 501 to 1,200 range; level II gives code letter J.
2. Table 2: code J means a sample of 80 dressers.
3. Read the accept and reject numbers for each class:

If the inspector finds 4 major and 6 minor defects in the 80 dressers, both classes are within their accept numbers. If 6 major defects are found, the major class reaches its reject number of 6 and the lot fails, even if the minor count is low. One critical defect fails the lot.

For a small lot, the arrows matter. A lot of 50 at level II is code D (8 samples), but the major plan at AQL 2.5 moves up to 5 samples (Ac 0, Re 1) and the minor plan at AQL 4.0 moves down to 13 samples (Ac 1, Re 2). The AQL calculator shows these moves for any lot.

AQL limits and misconceptions

• "AQL 2.5 means 2.5% defective is fine." AQL is a process average the plan treats as acceptable over many lots, not a promise about one lot.
• "An accepted lot has no defects." Defects found in the sample still exist and should be repaired or replaced, and the unsampled units contain some too.
• "Bigger lots need proportionally bigger samples." They do not. 80 samples cover every lot from 501 to 1,200 at level II.
• "A lower AQL is always better." It is stricter, not better. Too strict a plan fails lots you would accept and raises cost.
• "AQL covers everything." It covers the attributes on your checklist. Lab tests, packing and documents are checked separately.
• "Defects and defectives are the same." One unit can have several defects. Agree whether you count defective units or defects per hundred units.

How AQL results relate to the verdict

AQL gives two outcomes per defect class: accept or reject. Every Clear To Load report ends in one of three verdicts, which map onto the standard pass, pending and fail terms:

A report is Not cleared when any critical defect is found, when major or minor defects reach the AQL reject number, or when a safety or function test fails. It is issued as Hold when the AQL results are within limits but something needs your decision before shipping: a quantity shortfall, packing, labelling or marking that differs from your specification, a colour or finish difference from the approved sample, missing documents, goods reworked during the inspection, or an inspection that could not be completed in full. See the verdict definition.

Sources

• ISO 2859-1:2026 (3rd edition, January 2026), Sampling procedures for inspection by attributes, Part 1 (iso.org)

ANSI/ASQ Z1.4 (published by ASQ) uses the same code letters and plans for these columns. The standards are sold by ISO and ASQ; buy the edition your contract cites. Reviewed 14 Oct 2026.