Quality8 Oct 20265 min read

AQL 2.5 in plain numbers, and what it lets through

AQL 2.5 does not mean 2.5% defects are acceptable. It means something narrower and more useful, and once you can read the table you can tell whether your inspection report is real.

AQL 2.5 on a 3,200-unit batch means 125 pieces are pulled at random and up to 7 major defects are still accepted — a 5.6% defect rate in the sample. That sounds generous until you read what the plan is for: it is a screen against a supplier producing at 6% or worse, not a certificate that every piece is good.

An inspection report lands in your inbox with the line “AQL 2.5, Major 2, Minor 4, result: PASS” and a photograph of a carton. You are being asked to release a container on that basis. It is worth knowing what the number means, because it is narrower than most buyers assume and more useful than most buyers realise.

What AQL stands for

Acceptance Quality Limit. It is a sampling standard, originally military, now codified as ISO 2859-1. The number does not describe your goods. It describes the sampling plan: how many units get pulled from the batch, and how many defects among them still allow the batch to be accepted.

A 2.5 AQL on major defects does not mean “up to 2.5% of the order may be broken”. For a large batch it is close to that. For a small batch it is not, because the sample can only be as fine as the number of pieces you physically inspect.

How the sample size is set

Two inputs: the batch size, and the inspection level. General inspection level II is the default in most consumer goods. From those you read a code letter off the first table, then the sample size and the accept/reject numbers off the second.

Worked example, 3,200 units, level II, AQL 2.5 major and 4.0 minor:

Step	Result
Batch size	3,200 units
Code letter at level II	M
Sample size	125 units
Major defects, Ac / Re	7 / 8
Minor defects, Ac / Re	10 / 11

Meaning: of 125 pieces pulled at random, up to 7 major defects are accepted and 8 reject the batch. That is a defect rate in the sample of 5.6%, which sounds far too generous until you remember the batch is 3,200 and the plan is designed to catch a supplier who is producing at 6% or worse, not to certify that every piece is perfect.

This is the part buyers misread. AQL is a screen against a bad batch, not a guarantee of a good one. If your product is cosmetic and a 2.5% defect rate is commercially fatal, AQL 2.5 is the wrong plan and you need 1.0 or tighter, a bigger sample, or 100% inspection on the critical characteristic.

Critical, major, minor

The classification is where most of the practical value sits, because it is where the inspection either reflects your product or does not.

• Critical is a defect that makes the product unsafe or illegal: a live conductor exposed, a child’s item that fails a small-parts test, a CE mark on a product that does not comply. Critical is normally AQL 0, meaning one finding rejects the batch.
• Major is a defect that stops the product working or selling: a seam that fails, a hole pattern that does not line up, a wrong dimension outside tolerance.
• Minor is a cosmetic or packaging deviation that a customer would notice but not return.

If the checklist is written by the factory’s own quality manager, everything inconvenient lands in “minor”. A carton with the wrong shipping mark is minor until it reaches your customer’s warehouse and cannot be received. A tile with a 1.2mm thickness variance is minor until the installer cannot lay it flat.

What an inspector should be doing with a tape measure

An inspection that reports only visual findings is a photograph album. A real one measures: the dimensions on your drawing, the wall thickness, the weight per unit, the colour against a signed reference, the packing configuration against the carton artwork. Numbers, with tolerance and pass/fail per unit, not adjectives.

The inspection also has a timing problem. Checking finished goods is the last chance, and by then a rejected batch is expensive for everyone. The two earlier points are cheaper: first-off inspection before the run starts, and in-line checks at maybe 30% of production. A supplier who resists those two is telling you where their process control is.

How to read a report in five minutes

1. Check the batch and sample numbers. If 125 were inspected out of 3,200, the plan is real. If 20 were inspected out of 3,200, it is a visit, not an inspection.
2. Check the AQL lines against your PO. Suppliers and sometimes inspectors quietly use 4.0 for major. That is the difference between accepting 7 defects and accepting 10.
3. Find the measurements. If the report has no numbers with tolerances, the inspector did not have your drawing.
4. Look at what was rejected. A report with zero findings on a first production run is either a very good factory or an inspector who did not want an argument. Photographs of the actual defects settle it.
5. Check the seal. Container number, seal number, and photographs of the loading, matched to the packing list. This is the step that catches goods swapped after inspection.

None of it is complicated. It is the difference between paying for a document and paying for a decision. Our inspection sits inside the full-service arrangement, and the checklist is written from your specification before anyone goes to the factory.