AQL Basics


AQL stands for “Acceptable Quality Limit,” and it’s an important standard that’s used in the quality control industry. It’s defined in ISO 2859-1 as: “The quality level that is the worst tolerable,” over the course of many inspections.

Read more for an overview the use of ISO levels for sampling plans and AQLs for failure definitions.

AQL Sampling Plans & Standards

How to Determine the Sample Size?

Use the table below to determine the Sample Size Code. In an example scenario, consider working with General Inspection Level II (Inspection Level will be given by server and can be changed for any inspection level below at any time) and a lot size (aka. pallet size = how many boxes are on the pallet) of 120. The table below shows that this scenario is Sample Size Code F

With the sample size code F, reference the Sampling Plans table (above) to determine the Sample Size. Size F corresponds to 20 which means the QC will inspect a sample from the lot equal to 20 pairs of shoes.

What are the Defect Types?

Defects are organized into three category types by their severity:

  • Critical – usually indicates a defect that can harm people or cause injury. If the inspection fails on a critical defect, it is a failure regardless of the amount of Major or Minor defects.
  • Major – defect which influences the function of the product. If the inspection fails on a major defect, it is a failure regardless of the amount of minor defects.
  • Minor – usually visual defects.

How to Use the Accepted Quality Levels?

Once again, reference the Sampling Plan table which is included again below to find the Accepted Quality Level for the sample size. Use the Sampling standards (code F, size 20) and an example AQL for the 3 defect types (configured on the server, might change during the time and might be specific for every unit):

  • Critical AQL – e.g. 0.1
  • Major AQL – e.g. 2.5
  • Minor AQL – e.g. 4.0

Follow the F row and find the highlighted columns, which represent the AQL for the defect types, to see for the sample size (20) the amount of defects which will determine whether an inspection passes or fails:

  • Critical – only 0 defects accepted at this level
  • Major – only 1 defect accepted at this level
  • Minor – only 2 defects accepted at this level

When does the Inspection fail?

By Inspection, it is meant the whole process of inspecting the sample size (e.g. 20 products). All the defects must be counted together and compared with the AQL (explained above). Here are examples by category of the minimum defects required for failure (at sample size f-20):

  • The overall result of 1 Critical 0 Major 0 Minor => REJECTED (failed the Critical AQL)
  • The overall result of 0 Critical 2 Major 0 Minor => REJECTED (failed the Major AQL)
  • The overall result of 0 Critical 0 Major 3 Minor => REJECTED (failed the Minor AQL)