How to Read an AQL Chart (With a Worked Example)

How to Read an AQL Chart (and Use It on a Real Order)

An AQL chart looks cryptic the first time you see it: rows of lot sizes, columns of code letters, and a grid of Ac/Re numbers that seem to require a statistics background. In practice, the chart asks you only two questions, and both have a single answer. Step one: how big is the lot? Step two: what does the chart say about the sample and the accept/reject numbers? Everything else is context. This article walks through both steps, then applies them to a real order so you can fix the standard in your purchase terms before production starts.

The underlying standard is ANSI/ASQ Z1.4 / ISO 2859-1, using General Inspection Level II and normal single sampling. The values below are the ones we write into inspection plans by default. They do not change from order to order; what changes is the lot size, the product risk, and the level you agree with your supplier.

Step 1: From Lot Size to a Code Letter

The first column of the chart is the lot size, meaning the number of units in the batch presented for inspection. Read across that row until you reach the column for your inspection level. The cell you land on is a code letter: a letter from A to R, with the middle letters being the ones you will use most often.

Three general inspection levels exist: I, II and III. Level I takes smaller samples and is used when a larger risk is tolerable; Level III takes larger samples and is used when the cost of a defect is high. Level II is the default, and it is the level we apply unless your order terms say otherwise. The chart also has special levels S-1 to S-4, which take much smaller samples. Those are used when the test is destructive or expensive, for example pulling units apart to check a weld or running a drop test to failure. For routine pre-shipment inspection of consumer goods, you will work in the general levels.

Two practical points about lot size. First, the lot is the quantity actually presented for inspection, not the total quantity on the purchase order. If the factory presents 600 units out of an 1,800-unit order because the rest is still in production, the lot size is 600, and the chart is read on the 501-1,200 row. Second, lot size bands are ranges, not exact numbers, so a lot of 900 and a lot of 1,100 sit in the same band and take the same sample.

Step 2: From the Code Letter to Sample Size and Ac/Re

Once you have the code letter, the second table gives you three things: the sample size to draw, the acceptance number (Ac) and the rejection number (Re). Ac is the maximum number of defects you can find and still accept the lot. Re is the number at which you reject it. The two numbers are always consecutive, which is the point: there is no grey zone where the lot is neither accepted nor rejected.

The table below shows the bands you will meet most often on consumer goods orders, at General Inspection Level II, normal single sampling. The AQL 2.5 column is for major defects and the AQL 4.0 column is for minor defects.

Read the numbers as limits on the sample, not on the whole lot. If you draw 125 units and find 7 major defects, the lot is accepted under AQL 2.5, even though those 7 defective units are somewhere in the shipment. The chart is a sampling plan, not a quality guarantee, which is why the level you choose matters as much as the arithmetic.

A Worked Example: 900 Toys

Suppose you have ordered 900 plush toys and the factory reports that the full quantity is packed and ready. Here is the full read of the chart.

The lot size is 900. On the lot size column, 900 falls in the 501-1,200 band, which gives code letter J. On the sample size table, code J calls for a sample of 80 units. Those 80 units must be drawn at random from the presented lot, not selected by the factory from the top of the cartons.

Now apply the defect classes. Major defects are counted against AQL 2.5. The Ac/Re for code J at 2.5 is 5 / 6: if the inspector finds 5 or fewer major defects in the 80 units, the lot passes on that count; at 6, it fails. Minor defects are counted against AQL 4.0. The Ac/Re at 4.0 is 7 / 8: accept at 7 or fewer minor defects, reject at 8. The two counts are kept separate; a lot can pass on minors and fail on majors.

For toys, the inspection also carries category add-ons beyond the count: small-parts checks and EN71-related checks on components and labelling, plus a review of warning text and age marking. A safety hazard found during those checks is treated as a critical defect, which is the subject of the next section.

Major, Minor and Critical: Why Critical Is Not on the Chart

The chart has columns for AQL levels, but no column for critical defects, and that absence is deliberate. AQL is a negotiated allowance for defects that affect appearance, function or saleability within a defined limit. It is never a licence for a safety defect. A critical defect is a safety hazard or a regulatory violation: a detached small part on a toy for children under three, a live exposed wire, a prohibited substance in a food-contact item, a missing mandatory warning. One critical defect stops the lot, regardless of the sample count.

In practice, when a critical defect is found, the shipment is stopped and the batch is quarantined. The factory then runs a root-cause review using tools such as 8D, 5 Whys or a CAPA plan, and the batch is either re-made, corrected and re-inspected, or the supplier is changed if the cause is systemic. The distinction between the three classes drives the remedy as well: minor defects are handled on site by fixing, repacking or relabelling; major defects call for rework, replacement of shortfalls and re-inspection before release; critical defects stop the shipment.

Normal, Tightened and Reduced Inspection

ANSI/ASQ Z1.4 includes switching rules between three states. Normal inspection is the starting state: the sample sizes and Ac/Re numbers above apply as written. Tightened inspection applies when a supplier's quality history is poor, for example when two out of five consecutive lots have been rejected on original inspection. Under tightened inspection the same sample size is judged against stricter acceptance numbers, so it takes fewer defects to reject the lot. Reduced inspection applies when quality has been consistently good over a run of consecutive accepted lots; it uses smaller samples and is withdrawn as soon as a lot is rejected.

For a new supplier, or a supplier you have not used for this product before, the sensible starting point is normal inspection with tightened inspection held in reserve, and for higher-risk products, starting directly on tightened inspection. The switching rules are written into the inspection plan at the time the order is placed, not improvised after the first result arrives.

Choosing Your AQL Levels by Product Risk

The chart gives you the mechanics; the levels are your risk decision. The default applied is Major 2.5 / Minor 4.0, which suits general household goods, hardware, stationery, bags and similar categories where a defect is a cost problem rather than a safety problem. For children's products and food-contact items, the calculation changes. A defect that creates a choking hazard or a contamination risk is not a commercial inconvenience, so a stricter level such as Major 1.0 is appropriate, and critical defects are treated as stop-shipment events under any level. Stricter levels such as 0.65 or 1.0 are available on request and are written into the order terms.

Category risk also changes what the inspector looks for, not just the count. Textiles carry colourfastness, pH, formaldehyde and azo dye checks; hardware carries hardness, coating thickness and torque checks; kitchenware carries food-contact migration checks; small appliances carry function and safety-mark reviews. Those checks sit alongside the AQL count, and the plan should state which apply to your product before the inspection is booked.

Three Chart Mistakes

Using the production quantity instead of the lot presented. The chart is read on the quantity actually on the floor and ready for inspection. If 40% of the order is still in production, the lot is the finished portion, and the sample comes from that portion. Reading the chart on the purchase order quantity produces a sample that the factory cannot physically draw, and an inspection report that does not describe the goods that will ship.

Letting the factory pick the samples. The sample must be drawn at random across the lot, from multiple cartons and, where relevant, multiple production shifts. A factory that selects the units for inspection is selecting the result. Random selection is written into the plan, and buyers can follow the draw by live video if they want to see it happen.

Negotiating the AQL after seeing the result. The level, the sample size and the Ac/Re numbers belong in the order terms before production. Once a result is on the table, any change to the standard is a negotiation about the result rather than a quality decision. Fixing the standard in advance removes that conversation entirely.

Put the Standard in the Order Terms

The chart is only useful if it is fixed before the first unit is made. Write the inspection level, the AQL for major and minor defects, the critical-defect rule and the checkpoints into your order terms at the point of order placement, alongside the product specification. From then on, the chart does the arguing for you: the sample size is not an opinion, the Ac/Re numbers are not a negotiating position, and a lot that exceeds the limit is rejected on the numbers rather than on the relationship. The executing inspectors run in-process and pre-shipment checks against that plan; we coordinate any buyer-designated third-party agency or accredited (ISO 17025) laboratory where testing is required, and the inspection report - a PDF with photos, video, the defect list and re-inspection records - is issued by the party that performed the inspection and sent to you for confirmation at least three days before shipment. Test reports are issued by the laboratory in its own name and carry its report number; testing is arranged per product, per destination market, per batch. Certificates available upon request. To fix an inspection plan for your next order, contact [email protected].