AQL Inspection Explained: How Sampling Works and How to Choose Your Levels

AQL decides how many units an inspector checks and how many defects you accept before a shipment fails. Most buyers inherit "AQL 2.5" from a template without knowing what it means. Here is what it is, what it isn't, and how to set it for your products.

What AQL actually means

AQL stands for acceptance quality limit, the term used in ISO 2859-1. It is the worst defect rate you are prepared to tolerate as an average over a series of lots. Lots running at or below it will usually pass.

It is not a promise that a lot contains no more than 2.5% defects. A worse lot can pass by chance, and a better one can occasionally fail. AQL is a sampling plan: a trade-off between the cost of checking and the certainty you get.

The three numbers in every plan

1. Inspection level. Together with the lot size, it sets a sample-size code letter. General level II is the default. Level I takes a smaller sample, level III a larger one. Special levels S-1 to S-4 use small samples for slow or destructive checks.
2. Sample size. How many units the inspector draws, at random, from the whole lot.
3. Accept and reject numbers for each defect class. At or below the accept number, the lot passes for that class; at or above the reject number, it fails.

Sample sizes at general level II

Lot size	Code letter	Units sampled
151–280	G	32
281–500	H	50
501–1,200	J	80
1,201–3,200	K	125
3,201–10,000	L	200
10,001–35,000	M	315
35,001–150,000	N	500

Accept and reject numbers (single sampling, normal inspection)

Code letter	Units sampled	AQL 1.0: pass at / fail at	AQL 2.5: pass at / fail at	AQL 4.0: pass at / fail at
H	50	1 / 2	3 / 4	5 / 6
J	80	2 / 3	5 / 6	7 / 8
K	125	3 / 4	7 / 8	10 / 11
L	200	5 / 6	10 / 11	14 / 15
M	315	7 / 8	14 / 15	21 / 22

These are the values most inspection companies work from. ISO published a 2026 edition of ISO 2859-1, so confirm the tables your inspector uses.

A worked example

An order of 5,000 sets falls in the 3,201–10,000 band, so code L: 200 units are sampled. The plan is zero critical defects, AQL 2.5 for majors (pass at 10, fail at 11) and AQL 4.0 for minors (pass at 14, fail at 15).

The inspector finds no critical defects, 9 major and 16 minor. Majors pass, because 9 is within 10. Minors fail, because 16 reaches the reject number. A fail in any class fails the lot, so this shipment does not load until the minors are sorted and re-inspected.

How to choose your levels

• Critical defects: zero. Anything unsafe, illegal or certain to cause a return. One found means the lot fails.
• Major defects: 2.5 is the common default for consumer goods. Go to 1.5 or 1.0 for electrical items, premium retail, or marketplaces where one bad unit means a one-star review.
• Minor defects: 4.0 is common; 2.5 for premium ranges.
• The defect list matters more than the number. An AQL means nothing until each defect is assigned to a class for that product. See defect classification.

Special checks don't use the general sample

Measurements, function tests, drop tests and material checks such as XRF take time or destroy units, so they use special levels or fixed counts: for example five units for XRF, or one carton for a drop test. Write them into the inspection plan, or the inspector will choose for you.

Switching rules

ISO 2859-1 is designed for a continuing series of lots. After repeated failures it switches to tightened inspection; after a long run of passes it allows reduced inspection. Most importers simply use normal inspection every time. If you buy often from one supplier, tightening after failures is a fair and well-understood lever.

Mistakes that make AQL useless

• Sampling from the first carton or pallet. Units must come from cartons across the whole lot; a common rule is to open at least the square root of the number of cartons.
• Letting the factory pick the units.
• No written defect list, so the inspector judges on the day.
• Mixing very different SKUs into one lot. Inspect a high-risk SKU as its own lot.
• Treating AQL as a promise to your customer. It controls sampling risk, not the defect rate in every box.
• Leaving it out of the purchase order.

What to write in your PO and inspection booking

Inspection to ISO 2859-1, single sampling, normal inspection, general level II. Zero critical defects accepted; AQL 2.5 for major and 4.0 for minor defects. Special checks per the attached inspection plan. Defects classified per Specification v3.2.

FAQ

Is AQL 2.5 a quality grade? No. It is a common sampling setting, not a measure of how good the product is.

Can I rely on the factory's own AQL inspection? Their QC helps, but it isn't independent. The inspector should report to you.

Do the tables differ for double sampling? Yes. Double and multiple sampling plans use smaller first samples with a second stage. Single sampling is simpler and is what most consumer-goods inspections use.

Want inspections set up and read for you? See our quality control service and the pre-shipment inspection checklist.