If you have ever looked at an AQL inspection report, you have probably seen a "sample size code letter" somewhere on the form — often a single letter like "K" or "L" or "M". But what do these letters mean? Why does a lot of 1,500 units map to code letter J? And why does it matter? Understanding AQL sample size code letters is key to understanding how AQL sampling actually works.

What Are Code Letters?

Sample size code letters are a shorthand system used in the ISO 2859-1 and ANSI/ASQ Z1.4 standards to link lot (batch) size and inspection level to a specific sample size. Instead of having a separate table for every possible combination, the standards use code letters (A through R) as an intermediate step.

Here is how it works:

1. Determine your lot size (the total number of units in the batch)
2. Choose your inspection level (General I, II, III, or Special S-1 through S-4)
3. Look up the sample size code letter from Table A of the standard
4. Use the code letter to find the sample size and acceptance/rejection numbers for your chosen AQL levels

The code letter system makes the tables more compact and easier to use. Rather than a table with hundreds of lot size rows, there are only 16 code letters to deal with.

Code Letters by Lot Size and Inspection Level

The following table shows sample size code letters for common lot sizes at General Inspection Level II — the most commonly used level for consumer products:

• 2-8 units: Code letter A (2 samples)
• 9-15 units: Code letter B (3 samples)
• 16-25 units: Code letter C (5 samples)
• 26-50 units: Code letter D (8 samples)
• 51-90 units: Code letter E (13 samples)
• 91-150 units: Code letter F (20 samples)
• 151-280 units: Code letter G (32 samples)
• 281-500 units: Code letter H (50 samples)
• 501-1,200 units: Code letter J (80 samples)
• 1,201-3,200 units: Code letter K (125 samples)
• 3,201-10,000 units: Code letter L (200 samples)
• 10,001-35,000 units: Code letter M (315 samples)
• 35,001-150,000 units: Code letter N (500 samples)
• 150,001-500,000 units: Code letter P (800 samples)
• 500,001 and over: Code letter Q (1,250 samples)

At General Inspection Level I, you would drop down about one code letter level. At General Inspection Level III, you would go up about one level. For example, a lot of 2,000 units would be code letter J at GI, K at GII, and L at GIII.

Ac and Re Numbers for Common AQL Levels

Once you have the code letter and sample size, you can find the acceptance (Ac) and rejection (Re) numbers for each AQL level. Here are Ac/Re numbers for the most commonly used AQL levels at several common code letters:

Code letter J (80 samples, lot 501-1,200):

• AQL 2.5 (major): Ac=5, Re=6
• AQL 4.0 (minor): Ac=7, Re=8

Code letter K (125 samples, lot 1,201-3,200):

• AQL 2.5 (major): Ac=7, Re=8
• AQL 4.0 (minor): Ac=10, Re=11

Code letter L (200 samples, lot 3,201-10,000):

• AQL 2.5 (major): Ac=10, Re=11
• AQL 4.0 (minor): Ac=14, Re=15

Code letter M (315 samples, lot 10,001-35,000):

• AQL 2.5 (major): Ac=14, Re=15
• AQL 4.0 (minor): Ac=21, Re=22

Notice a pattern: the acceptance number does not scale linearly with sample size. Doubling the lot size does not double the number of allowable defects. This is because AQL is designed to give you consistent statistical confidence across different lot sizes.

Why This Matters: An Example

Let us say you have an order of 800 Bluetooth speakers. You are using General Inspection Level II with AQL 2.5 for major defects and AQL 4.0 for minor defects.

Step 1: Lot size of 800 falls in the 501-1,200 range → Code letter J

Step 2: Code letter J → 80 samples

Step 3: For AQL 2.5 major → Ac=5, Re=6. For AQL 4.0 minor → Ac=7, Re=8

So the inspector will check 80 speakers. If there are 5 or fewer major defects, the lot passes on major defects. If there are 6 or more major defects, it fails. For minor defects, 7 or fewer passes, 8 or more fails.

Now, what if the order is 8,000 speakers — ten times as many?

Step 1: Lot size of 8,000 falls in the 3,201-10,000 range → Code letter L

Step 2: Code letter L → 200 samples (not 800!)

Step 3: For AQL 2.5 major → Ac=10, Re=11

So even though the order is ten times larger, you only inspect 2.5 times as many samples (200 vs 80). And you allow about twice as many defects (10 vs 5), not ten times as many. This is because statistical confidence increases with sample size, but at a decreasing rate.

Common Pitfall: Percentage Thinking

The most common mistake buyers make is thinking about AQL in percentage terms. "AQL 2.5 means 2.5% defects, so 2.5% of 80 samples is 2, so 2 defects should be the limit." But as we saw above, the actual acceptance number for 80 samples at AQL 2.5 is 5, not 2.

Why the discrepancy? Because AQL is not a simple percentage calculation. It is based on statistical probability distributions (specifically, the binomial distribution and operating characteristic curves). The AQL value represents the quality level that has a high probability of being accepted — typically about 95% probability for lots at exactly the AQL quality level.

Best practice: Always use the official AQL tables or a trusted calculator — do not do percentage math in your head. If your inspection provider cannot tell you the code letter, sample size, and exact Ac/Re numbers for your order, that is a red flag. These are fundamental parameters of any AQL-based inspection, and they should be clearly stated on every report.

Special Inspection Levels

We have focused on General Inspection Levels, but it is worth mentioning the Special Inspection Levels (S-1, S-2, S-3, S-4). These use much smaller sample sizes and are reserved for specific situations:

• Destructive tests (where testing destroys the product)
• Very expensive or time-consuming tests
• Tests where failure is extremely rare

For example, you might use S-2 level for a drop test (since drops can damage the product), or S-3 for checking color consistency against a standard (since it is a quick check but you want more than just a couple of samples).

Special levels give you less statistical confidence but are necessary when full testing is impractical. They should only be used for specific tests within an overall inspection plan — not for the inspection as a whole.

Conclusion

AQL sample size code letters are the connective tissue of the AQL sampling system. They link lot size and inspection level to concrete sample sizes and acceptance criteria. Understanding how code letters work helps you make sense of inspection reports, verify that the correct sample size was used, and make informed decisions about inspection scope.

If you have questions about AQL sampling, code letters, or which inspection level is right for your products, contact QCANT at cs@QCANT.com. Our quality engineers can help you design an AQL sampling plan that balances cost, risk, and statistical confidence for your specific needs.