Reviewed November 2026. Formulas are shown above the guide. Report an error or request a calculator.
What AQL inspection decides
Checking every unit in a shipment is slow and expensive, so buyers and quality teams inspect a random sample instead. AQL sampling, defined in ISO 2859-1 and its American twin ANSI/ASQ Z1.4, tells you how big that sample must be and how many defective units it may contain before the whole lot is rejected.
The answer depends on three things: the lot size, the inspection level and the Acceptance Quality Limit you agreed for each class of defect. Critical defects make the product unsafe or illegal, major defects make it unlikely to be used or sold, and minor defects are cosmetic or small deviations.
Worked example
An order of 1,000 units at general inspection level II gives code letter J, which means a sample of 80 units.
| Defect class | AQL | Accept | Reject |
|---|---|---|---|
| Critical | 0 | 0 | 1 |
| Major | 2.5 | 5 | 6 |
| Minor | 4.0 | 7 | 8 |
If the inspector finds 2 major and 3 minor defects and nothing critical, every count is within its accept number and the lot passes. Six major defects would fail it even if everything else were perfect.
How the tables work
Table 1 of the standard converts lot size and inspection level into a code letter, and each letter has a fixed sample size, from 2 units (A) to 2,000 units (R). Table 2-A then gives the accept and reject numbers for each AQL. Where the table shows an arrow instead of numbers, you move to the nearest plan in the arrow's direction and use that plan's sample size. This calculator follows those arrows for you, which is why different AQLs can produce different sample sizes. In practice inspectors pull the largest sample and check it for all classes.
Common mistakes
- Reading AQL as the defect rate you will receive. AQL 2.5 does not mean the lot contains 2.5% defects; it is the worst average quality the plan accepts most of the time.
- Counting defects per unit wrongly. One unit with two major defects counts as one defective unit unless your agreement says otherwise.
- Changing the level after seeing results. Agree level II, or level III for a new supplier, before inspection.
Frequently asked questions
What AQL is standard for consumer goods?
A common choice is 0 for critical defects, 2.5 for major and 4.0 for minor, at general inspection level II.
What is the difference between level I, II and III?
Level II is the default. Level I uses smaller samples when quality history is good; level III uses larger samples for new suppliers or after problems.
Is ANSI Z1.4 the same as ISO 2859-1?
The sampling tables are practically the same for normal single sampling, so the sample sizes and accept numbers match.
What happens when the sample is larger than the lot?
Inspect every unit. The calculator caps the sample at the lot size and flags it.