An inspection report says “AQL 2.5 — PASSED, 5 defects found in 125 samples”. Many buyers accept that without knowing what it means — or that the same lot could contain far more defective units than they assume. Understanding AQL lets you set inspection criteria that match your real risk.
How AQL sampling works
AQL (Acceptance Quality Limit) sampling, defined in ISO 2859-1 (equivalent to ANSI/ASQ Z1.4), inspects a random sample from a lot and accepts or rejects the whole lot based on the number of defects found.
- Lot size — the number of units in the shipment.
- Inspection level — General Level II is the usual default; Level I inspects fewer units, Level III more. Special levels S-1 to S-4 are for expensive or destructive tests.
- Sample size code letter — found from lot size and inspection level.
- AQL per defect class — each class has its own acceptance (Ac) and rejection (Re) numbers for that sample size.
Worked example
Lot of 2,000 units, General Inspection Level II, normal single sampling: the lot size range 1,201–3,200 gives code letter K, which means a sample of 125 units.
| Defect class | AQL | Accept if defects ≤ | Reject if defects ≥ |
|---|---|---|---|
| Critical | 0 | 0 | 1 |
| Major | 1.0 | 3 | 4 |
| Major | 2.5 | 7 | 8 |
| Minor | 4.0 | 10 | 11 |
Each defect class is judged separately: the lot fails if any class reaches its reject number. Use the full ISO 2859-1 tables for other lot sizes.
Choosing AQL values
- Critical (safety risk, non-compliance, product unusable): zero accepted.
- Major (likely return or complaint): commonly 1.0–2.5; lower for premium or high-risk products.
- Minor (small cosmetic deviations unlikely to cause returns): commonly 2.5–4.0.
- Write a defect list with photos or limit samples so inspectors and the factory classify defects the same way.
What AQL does not do
- It does not guarantee the lot’s defect rate is at or below the AQL value — a sample can pass while the lot contains more defects.
- It does not find defects that the checklist does not look for.
- It does not replace process control: first article inspection, in-process checks and a stable process prevent defects; AQL only detects them.
Combine AQL inspection with the steps in our quality control guide. When you request a quote, tell us your required AQL levels so inspection is planned and priced from the start.