AQL Sampling Plan.
ANSI/ASQ Z1.4 and ISO 2859-1 single sampling plan. Enter your lot size and Acceptable Quality Level — get sample size, acceptance number, and rejection number for incoming or outgoing inspection.
Decision rule
If — or fewer defects are found → ACCEPT lot.
If — or more defects are found → REJECT lot.
Code letter (sample size code)
Sample size code letters (Level II — default)
| Lot size | S-1 | S-2 | S-3 | S-4 | Level I | Level II | Level III |
|---|---|---|---|---|---|---|---|
| 2 to 8 | A | A | A | A | A | A | B |
| 9 to 15 | A | A | A | A | A | B | C |
| 16 to 25 | A | A | B | B | B | C | D |
| 26 to 50 | A | B | B | C | C | D | E |
| 51 to 90 | B | B | C | C | C | E | F |
| 91 to 150 | B | B | C | D | D | F | G |
| 151 to 280 | B | C | D | E | E | G | H |
| 281 to 500 | B | C | D | E | F | H | J |
| 501 to 1,200 | C | C | E | F | G | J | K |
| 1,201 to 3,200 | C | D | E | G | H | K | L |
| 3,201 to 10,000 | C | D | F | G | J | L | M |
| 10,001 to 35,000 | C | D | F | H | K | M | N |
| 35,001 to 150,000 | D | E | G | J | L | N | P |
| 150,001 to 500,000 | D | E | G | J | M | P | Q |
| 500,001 and over | D | E | H | K | N | Q | R |
What is AQL Sampling?
AQL (Acceptable Quality Limit / Level) sampling is a statistical method for accepting or rejecting batches of parts without inspecting every piece. It's defined by ANSI/ASQ Z1.4 (formerly MIL-STD-105E) and used worldwide for incoming inspection, in-process, and final inspection.
How AQL Sampling Works
The standard maps lot size to a code letter (A through R), then maps the code letter and chosen AQL percentage to specific sample size and Ac/Re (Accept/Reject) values.
Lot size + Inspection level → Code letter
Code letter + AQL% → Sample size n, Ac (accept ≤), Re (reject ≥)
Inspection Levels Explained
- Level II — Default for normal inspection. Use this unless your contract says otherwise.
- Level I — Reduced inspection. Use when supplier has consistent track record.
- Level III — Tightened inspection. Use when supplier has had recent rejections.
- Special levels S-1 to S-4 — Small sample sizes for destructive testing or expensive inspections.
AQL Sampling Worked Example
You receive a batch of 500 machined parts. Inspection level II, AQL 1.0%.
- Lot size 500 → Code letter H
- Code H, AQL 1.0% → Sample size n = 50, Ac = 1, Re = 2
Decision rule: Inspect 50 random pieces. If 0 or 1 defects found → ACCEPT the lot. If 2 or more defects → REJECT the entire lot of 500.
AQL Levels for Different Industries
- 0.10% – 0.40% — Aerospace, medical devices, safety-critical.
- 0.65% – 1.0% — Automotive (Tata, Mahindra typical).
- 1.5% – 2.5% — Industrial machinery, general manufacturing.
- 4.0% – 6.5% — Consumer goods, low-criticality items.
Zero-Defect Sampling Plans
When Ac = 0, the plan rejects the entire lot on the first defect found. Common in aerospace and Tier-1 automotive. Train inspectors thoroughly — at Ac=0 there is no margin for misclassification.
Related Tools
Combine AQL inspection with supplier process capability data to reduce inspection burden over time. Document inspection plans in your PPAP submission. For risk-based prioritization of which features to sample, see FMEA RPN analysis. For aerospace FAI, link the sampling plan to specific characteristics in AS9102 Form 3.