What is AQL? The definition in the sampling standard

AQL is the number that, together with a sample size code letter, picks a sampling plan out of the tables. This page reads the definition as it is written in MIL-STD-105E, the public-domain standard from which ANSI/ASQ Z1.4 and ISO 2859-1 descend.

The definition

MIL-STD-105E spells AQL out as Acceptable Quality Level and defines it in clause 3.1: when a continuous series of lots is considered, the AQL is the quality level which, for the purposes of sampling inspection, is the limit of a satisfactory process average.

Three things in that sentence matter.

• A series of lots. The definition is about many lots from the same supplier over time, not one shipment.
• Process average. Clause 3.19 defines the process average as the average percent defective, or the average number of defects per hundred units, of product submitted for original inspection.
• A limit. The AQL marks the edge of what counts as a satisfactory average. It is not a target.

What the number tells you

The note to clause 3.1 says a sampling plan chosen for an AQL will accept the great majority of lots, provided the process average is no worse than the AQL. In the standard's words, the AQL is a designated value of percent defective (or defects per hundred units) for which lots will be accepted most of the time by the sampling procedure being used.

The same note adds two cautions:

• The probability of acceptance at the AQL depends on the sample size. It is generally higher for large samples than for small ones.
• The AQL alone does not identify the chances of accepting or rejecting an individual lot. To see those risks you have to look at the operating characteristic curve of the plan.

What the number does not tell you

Clause 4.4.2 is one sentence long: the selection or use of an AQL shall not imply that the contractor has the right to supply any defective unit of product. An AQL of 2.5 is not permission to ship lots that are 2.5 percent defective.

Clause 1.1 also sets the range. The plans apply to AQLs of 0.01 percent or higher, and the standard says they are not suitable where quality levels in the defective-parts-per-million range can be realized.

Percent defective or defects per hundred units

The standard counts nonconformance in two ways (clause 4.2):

• Percent defective counts units. A unit with three defects is one defective unit.
• Defects per hundred units counts defects. A unit with three defects adds three.

Clause 4.4.3 says AQL values of 10.0 or less may be expressed either way. AQL values over 10.0 are expressed in defects per hundred units only. That is why the table runs up to an AQL of 1000: at that end it is counting defects, not defective units.

One AQL per class of defect

Clause 4.4.3 allows different AQLs for groups of defects considered collectively, or for individual defects. The standard defines three classes:

• A critical defect is one that judgment and experience indicate would result in hazardous or unsafe conditions for people using, maintaining, or depending on the product (clause 3.5).
• A major defect is one, other than critical, that is likely to result in failure, or to reduce materially the usability of the unit for its intended purpose (clause 3.14).
• A minor defect is one that is not likely to reduce materially the usability of the unit, or is a departure from established standards with little bearing on its effective use (clause 3.16).

The calculator takes a separate AQL for each class and returns a plan for each.

Questions people ask

Is AQL a percentage of my shipment that may be defective?

No. Clause 3.1 defines it as a limit on the process average over a continuing series of lots, and clause 4.4.2 says choosing an AQL does not give a right to supply any defective unit.

Does a lot that passes meet the AQL?

Not necessarily. The note to clause 3.1 says the AQL alone does not identify the chances of accepting or rejecting individual lots. A sample can pass while the lot is worse than the AQL, and the size of that risk is read from the operating characteristic curve of the plan.

Why are there AQL values above 100?

Because above 10.0 the AQL is expressed in defects per hundred units (clause 4.4.3), and one unit can carry more than one defect.

Source

• MIL-STD-105E, Sampling Procedures and Tables for Inspection by Attributes, U.S. Department of Defense, 10 May 1989: clauses 1.1, 3.1, 3.5, 3.14, 3.16, 3.19, 4.2, 4.4.2 and 4.4.3. Page scans: archive.org/details/MIL-STD-105E_1. Read 11 October 2026.

Where your contract names ANSI/ASQ Z1.4, ISO 2859-1 or another standard, the wording of that edition is the one that applies.