Key points
- One sentence. AQL 2.5 means the inspection accepts a lot where no more than 2.5% of the sample are major defects — it is a sampling standard, not a zero-defect promise.
- Major vs minor. Major affects function or safety and stops the shipment; minor affects appearance only. They are counted separately, with separate limits.
- Sample size. Set by lot size under ISO 2859-1 general inspection level II — a 3,000-piece lot is checked at 125 pieces, not 3,000.
- On the report. Look for the lot size, sample size, accept/reject numbers per class, and the signature. A report without those four numbers is decoration.
AQL is the number that decides whether your container ships or sits in a warehouse in Shanghai while two people argue about who pays for a re-inspection. It is also the number most buyers nod at without knowing what it means.
The one-sentence version
AQL 2.5 means: in a lot of this size, we will accept a defect rate of up to 2.5% for major defects, and if the sample shows more than that, the whole lot is rejected.
Major, minor, critical
AQL is not one number, it is three.
- Critical — a defect that could injure someone or make the product illegal. Accept level is normally zero. One finding stops the shipment.
- Major — the product does not work, or fails in a way the customer will return. A clamp that does not close. A tray that does not fit the pole.
- Minor — cosmetic or packaging. A scratch on a hidden face, a print slightly off-centre. Usually 4.0.
How many pieces get checked
Sample size comes from a table (ISO 2859-1 / ANSI Z1.4), not from the inspector's mood. For a lot of 1,200 pieces at general inspection level II, the sample is 80 pieces. For 10,000 it is 200. You are not inspecting the lot — you are inspecting a sample large enough to make a statistically defensible decision about the lot.
| Lot size | Sample | Accept (major 2.5) | Reject |
|---|---|---|---|
| 151–280 | 32 | 2 | 3 |
| 281–500 | 50 | 3 | 4 |
| 501–1,200 | 80 | 5 | 6 |
| 1,201–3,200 | 125 | 7 | 8 |
Why not zero defects
Because zero-defect sampling at a reasonable sample size is not statistically achievable in a casting and assembly process — you would need to inspect 100% and still miss things. AQL 2.5 is a commercial compromise: it accepts that a small number of defects will reach the market, and it prices the warranty cost of that into the unit price.
If your channel cannot tolerate any returns — Amazon, for instance, where a 3-star review costs more than the part — then specify AQL 1.0 major / 2.5 minor and expect to pay 3–5% more. That is the honest trade.
Ask for the inspection report, not the certificate of inspection. One tells you what was found. The other tells you an inspection happened.
What to check on the report
- The lot size matches your PO, not the factory's production run.
- The sample size matches the table for that lot.
- Defects are listed individually, not summarised as "5 minor".
- There is a photo of every major defect.
- The inspector's name and date are on it.
