An inspector who checks an order of 3,000 lamps does not open all 3,000 boxes. Under the standard sampling table, they pull 125 at random. If you agreed AQL 2.5 for major defects, the batch passes with up to 7 major defects in those 125 and fails at 8. That one line of the AQL table decides whether you release the final payment to your supplier.
The table looks intimidating, but you only need two steps to read it. This article walks through them, explains which AQL to choose for which kind of defect, and shows where AQL stops protecting you.
What AQL means
AQL stands for Acceptance Quality Limit, the term used in ISO 2859-1 since its 1999 edition; older documents and many suppliers still say acceptable quality level. It is the worst average defect rate you are prepared to accept from a production process, expressed as a percentage. The AQL table then turns that percentage into a practical sampling plan: how many units to check, and how many defects are allowed before the batch is rejected.
Because only a sample is checked, AQL is a statement about risk, not a guarantee. A batch that passes can still contain defective units, which is why the choice of AQL and the definition of a defect matter as much as the table itself.
How to read the AQL table in two steps
Step one: look up your lot size and inspection level to find a code letter. Unless agreed otherwise, the standard uses general inspection level II. Step two: look up that code letter to find the sample size, and read across to your AQL to find two numbers: Ac (accept) and Re (reject). If the number of defects found is at or below Ac, the batch passes; at or above Re, it fails.
The table below combines both steps for general inspection level II, single sampling, normal inspection, at the three AQL values used most for consumer goods.
| Lot size (units) | Code letter | Sample size | AQL 1.0 (Ac / Re) | AQL 2.5 (Ac / Re) | AQL 4.0 (Ac / Re) |
|---|---|---|---|---|---|
| 281 to 500 | H | 50 | 1 / 2 | 3 / 4 | 5 / 6 |
| 501 to 1,200 | J | 80 | 2 / 3 | 5 / 6 | 7 / 8 |
| 1,201 to 3,200 | K | 125 | 3 / 4 | 7 / 8 | 10 / 11 |
| 3,201 to 10,000 | L | 200 | 5 / 6 | 10 / 11 | 14 / 15 |
| 10,001 to 35,000 | M | 315 | 7 / 8 | 14 / 15 | 21 / 22 |
For smaller and larger lots some cells in the standard contain an arrow instead of numbers. The arrow sends you to the nearest plan above or below, and you then use that plan's sample size as well. If the sample size is as large as the lot, every unit is inspected.
Critical, major and minor defects
An inspection does not use one AQL for everything. Defects are sorted into three classes, each with its own limit. A critical defect makes the product unsafe or non-compliant, such as a sharp edge on a toy or a missing CE marking. A major defect makes the product likely to be returned or unsellable, such as a switch that does not work. A minor defect is a small cosmetic flaw that does not affect use.
For consumer goods a common setting is 0 for critical defects, 2.5 for major and 4.0 for minor. The zero is not a value from the ISO table but a contract choice: one critical defect fails the batch. Products with a higher risk, or customers with stricter returns policies, often use 1.0 for major defects.
A worked example
Take the 3,000 lamps from the introduction. The lot falls between 1,201 and 3,200, which gives code letter K and a sample size of 125 under level II. With 0 for critical, 2.5 for major and 4.0 for minor, the inspector checks the 125 units against your defect list.
| Defect class | AQL | Batch passes with | Batch fails at |
|---|---|---|---|
| Critical | 0 | 0 defects | 1 defect |
| Major | 2.5 | 7 or fewer | 8 or more |
| Minor | 4.0 | 10 or fewer | 11 or more |
Each class is judged on its own. Six major defects and twelve minor defects in this sample means the batch fails on minor defects, even though it would pass on major ones.
Want this applied to your order? Our inspectors work in sixteen countries and carry out a Pre Shipment Inspection against the AQL you agree with your supplier, with the photo report on the same working day. An inspection costs €250 per location per day.
Inspection levels and switching rules
Level II is the default, but the standard has others. Level I uses smaller samples and level III larger ones, for when you want less or more certainty. The special levels S-1 to S-4 use very small samples, meant for destructive or expensive tests such as cutting open a product or running a drop test.
ISO 2859-1 also describes tightened inspection, with usually the same sample size but lower acceptance numbers after repeated rejections, and reduced inspection, with smaller samples after a run of accepted batches. These switching rules only apply to a continuing series of lots from the same supplier. For a one-off order from a new supplier, normal inspection at level II is the usual starting point.
What AQL does not do
AQL only works if both sides use it. A factory's own quality check does not have to follow ISO 2859-1. If you want AQL to apply, require it in the order and have it checked by a third party. Put the AQL values and your defect list in the purchase order before production starts, not in an email after the goods are packed.
A sample at the end also cannot fix a problem that started on day one. A First Article Inspection checks materials and the first pieces off the line, and a During Production Inspection catches problems while there is still time to correct them. For products where any defect is unacceptable, such as safety-critical items, a 100% Inspection checks every unit instead of a sample. For an overview of all inspection types, see our guide to quality control.
Checklist before your next inspection
- Agree the AQL for critical, major and minor defects with your supplier, in writing, before production.
- Write a defect list per product that says which faults count as critical, major or minor.
- Use general inspection level II and single sampling unless you have a reason to change it.
- Look up the sample size and the accept and reject numbers for your lot size, and give them to the inspector.
- Decide in advance what happens when a batch fails: rework, sorting, a re-inspection or a price reduction.
- Hold back the final payment until the inspection report is in.
Not sure which AQL fits your product? Send us your product details through our contact page and we will tell you what we would use.
Sources
- ISO 2859-1, Sampling procedures for inspection by attributes, Part 1: Sampling schemes indexed by acceptance quality limit (AQL) for lot-by-lot inspection, Tables 1 and 2-A (single sampling, normal inspection). The values above follow the 1999 edition and the equivalent ANSI/ASQ Z1.4 tables; the current third edition (2026) is listed on iso.org
- ANSI/ASQ Z1.4, Sampling Procedures and Tables for Inspection by Attributes, the American standard with the same tables.
