Inspection Level in AQL Sampling: I, II, III, S-1 to S-4

What is an inspection level in AQL sampling?

An inspection level sets how large the sample is for a given lot size under ISO 2859-1. General levels I, II and III take small, standard and large samples; level II is the default. Special levels S-1 to S-4 take much smaller samples for slow, costly or destructive checks such as drop tests.

Last reviewed 14 Oct 2026 · Written by Renu Rajpurohit

One lot at five levels

The level changes the code letter, and the code letter changes the sample. For a lot of 2,000 units at AQL 2.5 for major defects:

Level	Code letter	Sample	Major Ac / Re at AQL 2.5
General I	H	50	3 / 4
General II	K	125	7 / 8
General III	L	200	10 / 11
Special S-4	G	32	2 / 3
Special S-3	E	13 (20 after the arrow)	1 / 2

At S-3, row E has an arrow in the AQL 2.5 column, so the plan moves down to row F: 20 units, accept 1, reject 2. The arrow rule is explained in AQL sampling explained.

Choosing a level

A larger sample tells good lots from bad ones more reliably, but takes longer on site. Level I suits low-risk goods from a supplier with a clean record. Level III suits a new supplier, a product with a history of problems, or the reinspection of a lot that failed. Special levels are for checks you cannot repeat 125 times in a day: a carton drop test, a full assembly trial or a detailed measurement of every dimension.

One inspection can use two levels. A furniture order might take general level II for the visual check and S-2 for assembly trials.

How Clear To Load sets the level

Unless you choose otherwise, we use general level II, and we name the levels in the checklist before the visit. Because sample size drives time on site, the level also affects how many man-days an order needs; we confirm that before you book. Moving from level II to III on a 2,000-piece order adds 75 units to check.

Sources

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

Frequently asked questions

Is level II always the right choice?

It is the standard default, not a rule. Use level III where a failure would be costly, and level I where the supplier has a long clean record and you accept less certainty.

Do special levels give weaker protection?

Yes. Smaller samples catch fewer problems. That is acceptable for slow or destructive checks, because the main visual inspection still runs at a general level.

Does the level change the AQL?

No. The level sets the sample size; the AQL sets the accept and reject numbers in that sample. You choose both.