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AQL Sample Size Code Letter Reference Table (ANSI/ASQ Z1.4)

This is the lot size to code letter lookup from Table I of ANSI/ASQ Z1.4, with the single sampling sample size next to each letter. Search by lot size or by letter and export the table for your inspection instructions.

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Lot or batch sizeLevel I codeLevel I sampleLevel II codeLevel II sampleLevel III codeLevel III sample
2 to 8
A
2
A
2
B
3
9 to 15
A
2
B
3
C
5
16 to 25
B
3
C
5
D
8
26 to 50
C
5
D
8
E
13
51 to 90
C
5
E
13
F
20
91 to 150
D
8
F
20
G
32
151 to 280
E
13
G
32
H
50
281 to 500
F
20
H
50
J
80
501 to 1,200
G
32
J
80
K
125
1,201 to 3,200
H
50
K
125
L
200
3,201 to 10,000
J
80
L
200
M
315
10,001 to 35,000
K
125
M
315
N
500
35,001 to 150,000
L
200
N
500
P
800
150,001 to 500,000
M
315
P
800
Q
1,250
500,001 and over
N
500
Q
1,250
R
2,000

General inspection levels only. The standard also has special levels S-1 to S-4 for destructive or costly tests; see Table I in your edition. Sample size shown is the single sampling size for the code letter. Accept and reject numbers depend on the AQL and are in Tables II-A, II-B and II-C of the standard.

Store AQL and inspection level by supplier

Optional: we can review where sampling plans live in your ERP and how to make them part of the receiving transaction.

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How to use the table

Find the row that contains your lot size, then read across to the column for your inspection level. Level II is the default. The code letter gives the sample size in the next column. You then take that letter to the master tables of the standard (II-A for normal, II-B for tightened, II-C for reduced inspection) and read the accept and reject numbers under your AQL. The search box filters rows, so typing a letter such as K shows every lot range that uses it.

  • Use the lot size of the lot actually presented for inspection.
  • If a lot falls on a band edge, such as exactly 500 or 501 units, check which band it belongs to.
  • Keep the same level for a supplier unless the contract changes it.

Where the letters come from

The letters A to R, with I and O omitted, run in order of increasing sample size from 2 units to 2,000 units. Larger lots get a larger sample, but the increase is less than proportional, which is why the percentage of a lot inspected falls as lots get bigger. Level III roughly shifts each lot band one letter up from Level II, and Level I shifts it down, so Level III gives better discrimination between good and poor lots at a higher inspection cost.

Using the letter with your AQL

The code letter alone does not decide whether a lot passes. For example, letter K has a sample of 125. At AQL 1.0 the normal plan accepts up to 3 defects and rejects at 4, while at AQL 2.5 it accepts up to 7 and rejects at 8. Different AQLs for critical, major and minor defects are normal, so the same sample is often judged against several limits. Document the AQL for each defect class on your work instruction and on the inspection report.

Common mistakes

Inspectors sometimes read the sample size from a table for a different level, or round the lot size to the nearest hundred and land in the wrong band. A second error is using the table for variables data. Z1.4 is an attribute plan: units are classified as conforming or nonconforming, or defects are counted. Check that your customer has not specified a different edition or a specific plan before using general tables.

Frequently Asked Questions

What is the most commonly used AQL inspection level?

General inspection Level II is the default in Z1.4 and the most common choice for incoming goods. Level I is used when less discrimination is acceptable, and Level III when more is needed. Special levels S-1 to S-4 are for small samples where sampling cost or destructive testing matters.

Why do some lot sizes jump two letters between levels?

The levels are not a fixed multiple of one another. At the small end Levels I and II share letter A and at the other end the spacing differs. Always read the row directly instead of estimating by adding or removing a letter.

Does the table change for tightened or reduced inspection?

The code letter and sample size stay the same for single sampling in tightened inspection. The accept and reject numbers change. Reduced inspection in Z1.4 uses a smaller sample for most plans, so check Table II-C. The switching rules decide which table applies.

Is this the same table as ISO 2859-1?

Yes for the general levels. ISO 2859-1 and ANSI/ASQ Z1.4 share the code letters and sample sizes. Differences between them are mainly in presentation and in the terminology used. Confirm the edition your customer requires if the contract names one.

If you want sampling levels and AQLs stored by supplier and part in your ERP, we can show how to drive inspection lot creation from that data.

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