October 8, 2026
·By Ryo Shibata, CTO
AQL Sampling for Incoming Inspection
AQL, or acceptable quality limit, is the maximum defect rate in a delivered lot that you are willing to treat as acceptable. In incoming inspection it drives a sampling plan: instead of checking every unit, you inspect a defined sample from the lot, count the defects, and accept or reject the whole lot based on a pass number set in advance. It lets a small team check large deliveries without either inspecting everything or inspecting nothing.
Why sample at all
Inspecting every unit of every delivery is rarely practical, and it is not even perfectly reliable, because people miss defects when they check thousands of identical items. Not inspecting at all means the first time you learn about a bad lot is when a customer returns it. Sampling sits in the middle. It gives you a statistically grounded decision rule at a fraction of the effort. If you are not inspecting today, the inspection step most teams skip explains what that gap costs.
The three inputs
A sampling plan is built from three things:
- Lot size. How many units are in the delivery you are judging.
- Inspection level. How strict the inspection is. General level II is the usual default. Tighter levels inspect more units, looser levels inspect fewer.
- The AQL value. The defect rate you will tolerate, expressed as a percentage, such as 1.0 or 2.5.
Together these point to a sample size and two numbers: the accept number (the most defects you can find and still pass the lot) and the reject number (the count at which you fail it). The tables come from standards such as ISO 2859-1 and ANSI/ASQ Z1.4, which are the reference you should check before adopting any plan.
A worked example
You receive a lot of 2,000 units. Using general inspection level II, a lot of 1,201 to 3,200 units falls under sample size code letter K, which calls for a sample of 125 units. Taking the single sampling plan for normal inspection, the accept and reject numbers depend on your AQL:
| AQL | Accept | Reject |
|---|---|---|
| 1.0 | 3 | 4 |
| 2.5 | 7 | 8 |
At an AQL of 1.0, you inspect 125 units. Find 3 or fewer defective units and the lot passes. Find 4 or more and it is rejected. At 2.5 you are more tolerant, passing the lot with up to 7 defects. These values come from the published tables for this lot size and level. Verify them against the current standard before you use them in a real plan, since your supplier agreement may specify a different standard or version.
Defect classes: major, minor, critical
Most plans set separate AQL values by how serious a defect is:
- Critical. A defect that makes the product unsafe or unusable in a way that creates legal or safety risk. The AQL is typically 0, and critical defects often trigger full inspection regardless of the sample result.
- Major. A defect that makes the product fail its function or that a customer would reject. A common AQL is 2.5.
- Minor. A cosmetic or small deviation that does not affect function. A common AQL is 4.0, a looser tolerance.
Writing these definitions down with photos or examples before the first inspection avoids disputes with suppliers later, because “defective” stops being a matter of opinion.
What happens when a lot fails
A failed lot needs a defined response, decided in advance:
- Reject and return the lot to the supplier.
- Sort or rework the lot, inspecting 100 percent and removing the bad units, with the supplier bearing the cost.
- Accept with a concession, documented, when you need the stock and the defect is minor.
Whatever you choose, log it. A single failed lot is a data point. Repeated failures from the same supplier are a pattern, and a pattern is when you open a formal corrective action. The CAPA process covers what to do once a failure stops looking like a one-off. You also need a clear defect rate measurement so the numbers you log are comparable across lots and suppliers.
Sampling limits to keep in mind
Sampling is a decision rule, not a guarantee. A lot that passes can still contain defects, up to the AQL level you accepted. That is the trade you are making. It also assumes the sample is random and representative. Pull units from across the whole lot, not just the top carton, because defects often cluster by pallet or production run. If a lot is later found to be bad, your records of what you inspected are part of batch recall readiness.
Where Flowbound fits
Flowbound does not set your AQL or choose your sampling plan, that stays a quality decision for your team and your supplier agreement. What it handles is the tracking. Inspection Tracking logs incoming inspection results and flags anything that fails before it reaches your shelf. Quality Monitoring watches defect patterns across returns and inspections, and keeps a running defect rate and on-time-in-full record for each supplier, so a single failed lot gets noticed alongside the history behind it. If you are comparing suppliers, those records feed a supplier scorecard.
FAQ
What does AQL stand for? AQL stands for acceptable quality limit. It is the maximum percentage of defective units in a lot that you are prepared to treat as acceptable, and it determines the pass and fail numbers in a sampling plan.
How do I find the right sample size? You need the lot size, the inspection level, and the AQL. The lot size and level give a sample size code letter, and the standard’s tables, such as ISO 2859-1, give the sample size and the accept and reject numbers. For a 2,000-unit lot at general level II the sample is 125 units. Pairing that with a clear defect rate definition keeps results consistent.
Is a lower AQL always better? A lower AQL is stricter, so it rejects more lots and pushes more cost back to the supplier. That can be right for critical products, but unnecessarily strict tolerances raise your own cost and strain the relationship. Match the AQL to the real risk of the defect.
Does passing an AQL inspection mean the lot is defect-free? No. It means the sample stayed within the accept number, so the lot is statistically likely to be at or below your tolerated defect rate. Some defects can remain. Keep inspection records for recall readiness.
Can software run the inspection for me? Software cannot physically inspect units, but it can log results, flag failures, and track patterns by supplier. Quality Monitoring does that, so you see trends instead of isolated results.