AQL Sampling Explained: How to Read an Inspection Plan Before You Pay

In February 2023 a thread on Hacker News carried the headline "50% rejection rate for iPhone casings produced in India" (Hacker News, 2023 (opens in a new tab)). It's a scary number, and the first useful reply wasn't about India at all. An engineer who qualifies suppliers wrote that "when first qualifying a supplier, I expect IQC to fairly exhaustively test every incoming part", which is to say, heavy checking on a new supplier is normal anywhere. A rejection rate means nothing until you know how many parts were checked, against which limit, and what counted as a defect.

That's what AQL is for. It's the language every inspection firm uses to turn "check my order" into a pass or a fail, and most buyers sign off a booking without reading it. This guide shows you how to read an AQL plan, how to choose one, and how to write it into your purchase order so the inspector works to your rules and not the firm's defaults. One caveat first - the standard's tables are copyrighted, so we explain the logic and one worked example, and you should read the numbers for your own lot off the standard or your inspection firm's report.

1. What An AQL Actually Is

AQL stands for acceptable quality limit. It's the worst defect rate you're prepared to accept in a lot, set separately for each class of defect. The inspector doesn't check every piece. A sampling plan tells them how many to pull and the most defects they may find before the lot fails.

The plan comes from a standard. ISO 2859-1 is the international one. Its US cousin, ASQ/ANSI Z1.4, descends from the old military standard MIL-STD-105 and uses the same logic (Quality Magazine (opens in a new tab)). Most inspection firms work to one or the other, and they describe the result as "AQL 2.5 for majors, 4.0 for minors" or similar. A plan has three parts: a sample size, an accept number and a reject number. Find the accept number of defects or fewer, and the lot passes. Find the reject number or more, and it fails.

Here's the part that trips people up. AQL 2.5 does not mean "2.5% of my goods may be bad". It's a label for a row in a table. In QIMA's worked example a 4,000 unit lot gets a 200 piece sample, and at AQL 2.5 the lot passes with 10 defects or fewer and fails at 11 (QIMA (opens in a new tab)). Ten defects in 200 is 5% of the sample. Same label, double the number. The AQL is the quality level the plan is designed to protect, and the lots it lets through can look worse than the label when you count what was in the sample.

2. Lot Size, Inspection Level And Sample Size

Three choices fix the sample size, and only the third is really yours.

First, the lot size, which is the quantity the inspection covers. Second, the inspection level. General Level II is the default for most consumer goods. Level I uses fewer pieces and suits a supplier with a strong record, with more risk of missing a problem. Level III uses more pieces and fits a supplier with recent quality trouble (QualityInspection.org (opens in a new tab)). There are also special levels S-1 to S-4 for cases where small samples are all you can afford, such as destructive tests, time-consuming checks and container-loading supervision (same source). The lot size and the level together give a code letter, and the code letter gives the sample size.

Third, the AQL for each defect class, which decides the accept and reject numbers. A looser AQL like 4.0 allows more defects than a tighter one like 1.5, on the same sample.

Now look at what the sample size does as orders grow. At Level II, 4,000 pieces gives a 200 piece sample, 5,000 gives 200 as well, and 40,000 gives 500 (QIMA (opens in a new tab), QualityInspection.org (opens in a new tab)). Our arithmetic on those figures is that the inspector looks at 5% of the 4,000 piece lot and 1.25% of the 40,000 piece lot. So the bigger your order, the thinner the check, which is why buyers of large runs usually split them into separate lots by colour, style or production week and have each lot sampled. The lot is something you define in the booking. If one lot hides two fibre sources or two factories, the sample can't tell you, and that's the exact failure described on quality control in India.

3. Critical, Major And Minor

AQL is set per class of defect, and the classes carry most of the meaning.

A critical defect is one that makes the product unsafe or illegal. Inspection firms commonly set the AQL for critical defects to zero, because critical defects are unacceptable (QIMA (opens in a new tab)). In clothing that means a broken needle left in a garment or a missing legal label. In leather goods or metals it can mean a failed restricted-substance result. One critical defect in the sample fails the lot.

A major defect affects function or appearance enough that the buyer would likely return the product. The usual default is AQL 2.5. A minor defect is a small cosmetic issue that doesn't affect function, and the usual default is AQL 4.0 (same source).

Those defaults belong to the inspection firm, not to you. The defect list does too, unless you write one. A generic list won't know that a visible crooked bar-tack is a major defect on your brand and a shrug on someone else's. Leather shows how wide the gap can be. A seller on a forum defined "Grade A" as allowing one or two small defects in the most visible part of the hide, and the buyer expected almost none (r/Leathercraft, 2025 (opens in a new tab), Reported). Neither was dishonest. They hadn't agreed what a defect was. Put photos of each defect class in the order, and the argument never starts.

4. A Worked Example

Say you order 4,000 cotton T-shirts from a knit maker, such as one in Tiruppur, and book a pre-shipment inspection at General Level II. The plan is set at AQL 0 for critical, 2.5 for major and 4.0 for minor.

At 4,000 pieces the code letter is L and the sample is 200 pieces, drawn at random from across cartons, sizes and colours (QIMA's example uses this same lot size and sample). For majors at AQL 2.5 the plan accepts 10 and rejects at 11. The inspector opens cartons, pulls 200 shirts and sorts what they find into the three classes. The accept and reject numbers for critical and minor come from the same table at their own AQL, and your inspection firm's report prints all three.

Run three outcomes. The inspector finds no critical defects, seven majors and a handful of minors within the limit. The lot passes, and you pay the balance. Now suppose they find 11 majors, perhaps shoulder seams twisted on one production line. The lot fails, even though 189 of the 200 shirts were fine. Or suppose they find one critical, a missing care and fibre label, and nothing else. The lot fails on that alone, because the critical limit is zero.

What happens after a fail is where the contract earns its keep. The usual routes are a full sort by the factory and a re-inspection, a negotiated discount on a lot you choose to accept, or a rejected lot. None of those works unless your order says in advance who pays for the sort and the second inspection, and that the balance waits for a passing report. If you'd rather not run that process alone, SourcingSync (opens in a new tab) books the inspection, reads the report with you and handles the factory conversation.

5. What AQL Can't See

An AQL counts defects an inspector can see, count and measure on a table. It says nothing about several things that cost buyers real money.

It can't see a fabric that is lighter than quoted. A Gurugram production agent described factories reaching the quoted weight of heavy knits with a stiffener that washes out, so a sample that feels right can be a lighter cloth (r/streetwearstartup, 2026 (opens in a new tab), Reported, and he sells production services). It can't see shrinkage that appears after the first wash. The same agent says small orders are sometimes cut straight off the roll without a rest and shrink 5 to 8%, and he writes a 3% limit and a post-wash measurement into his orders. And it can't see who made the goods. He also says small bulk orders can be quietly moved to a cheaper unit than the sampling room. Job work is ordinary business in India, and not a fault in itself, but the unit you sampled and the unit you inspected may differ. Ask which unit makes your order before you book.

Chemistry is another blind spot. Pesticides, Salmonella, chromium in leather and cadmium in jewellery need a lab, not a visual check, and your border sets the test. That's on quality control in India, with the failure counts behind it.

The fix for each of these is a separate line in the order, not a tighter AQL. Fabric weight and shrinkage get their own test and limit, the production unit gets named, and lab tests get their own sample drawn by your inspector.

6. Tighten For New Suppliers, Relax For Proven Ones

The standard builds this in. ISO 2859-1 has three states, normal, tightened and reduced inspection, with switching rules that move you between them depending on how recent lots performed. The 2026 edition adds skip-lot sampling to those rules, so a supplier with a long clean record can have some lots skipped (Quality Magazine (opens in a new tab), Supported). We haven't read the switching tables themselves, so use your inspection firm's current version.

You don't need the formal rules to use the idea. Our reading, which is advice and not a finding, is this. On a first order from a new supplier, book Level III and keep the critical AQL at zero. Treat it as the price of learning how the factory behaves, in the spirit of the Hacker News engineer who expects to test almost everything at first. If the first two or three orders pass cleanly, move to Level II. Go to Level I or to occasional checks only when the record is long. If a lot fails, go back up a level for the next few orders. That's how you keep checking costs in proportion to what the supplier has earned.

7. What To Write In The Purchase Order

An inspection works for you only if it works to your numbers. The order, or the inspection booking attached to it, should say these things:

• The lot, and how it's split if the order has several colours, styles or material lots.
• The sampling standard and edition, the inspection level, and the AQL for critical, major and minor.
• A defect list with photos for each class, written for your product and your brand.
• That the inspector draws the sample at random from across cartons, never from a stack the factory has set out.
• The separate tests that AQL can't do, such as fabric weight, shrinkage and lab tests, each with its limit.
• What happens on a fail, who pays for a sort and re-inspection, and that the balance is released only against a passing report.

Start from an approved sample, because that's the reference an inspector compares to. The sample evaluation checklist covers that step, and the product sourcing process shows where inspection sits among the other stages.

Questions Buyers Ask

Back To The 50%

So what was that rejection rate telling anyone? On its own, nothing. It could mean a poor supplier, or a new line being qualified under exhaustive checks, which is exactly what the engineer in the thread described. The only way to know is to ask the questions this guide sets out: how many were checked, at what level, against which AQL, and with what defect list.

That's what you should be able to say about your own order before the inspector arrives. Name the lot, the level, the three AQLs and the defects, and put the tests AQL can't do on separate lines. Then a pass or a fail means something you agreed in advance, and not something an inspection firm decided on its own.

Where this fits in a country context, with the lab tests and inspection layer for Indian goods, is on quality control in India.

Related: Sample evaluation checklist · The product sourcing process · Quality control in India