AQL 2.5 Sampling Plans & Code Letter Determination
Lesson Contents Overview
Learning Objectives
Comprehensive Technical GuidanceTextbook & Training SOP
AQL 2.5 Sampling Plans & Code Letter Determination
═══════════════════════════════════════════════════════════════════════════════ MODULE 02: AQL 2.5 SAMPLING & INSPECTION STANDARDS LESSON 02.01: AQL 2.5 SAMPLING PLANS & CODE LETTER DETERMINATION ═══════════════════════════════════════════════════════════════════════════════ [Course: AQL 2.5 Quality Inspection & QA Masterclass] [Next: 02.02 Switching Rules ➡️]
1. Introduction: The Statistical Architecture of Apparel Auditing
In modern apparel manufacturing, where export purchase orders range from a few hundred pieces to several hundred thousand units, 100% finished garment inspection is both economically unfeasible and operationally impractical. Inspecting every single seam, stitch, button, and zipper on 100,000 shirts would create massive production bottlenecks, drive labor costs through the roof, and still fail to guarantee zero defects due to inevitable human inspection fatigue.
To resolve this challenge, the international ready-made garment (RMG) trade utilizes statistical acceptance sampling governed by the ISO 2859-1 and ANSI/ASQ Z1.4 standards.
At the core of this system is the Single Sampling Plan for Normal Inspection, pairing an Acceptable Quality Limit of 2.5 (AQL 2.5) for Major defects, AQL 4.0 for Minor defects, and AQL 0.0 (Zero Tolerance) for Critical safety defects.
┌─────────────────────────────────────────────────────────────────────────────┐ │ AQL 2.5 SINGLE SAMPLING PLAN ARCHITECTURE │ │ │ │ [STEP 1: LOT DEFINITION] ────► [STEP 2: CODE LETTER] ───► [STEP 3: LIMITS] │ │ Total Finished & Packed Cross-reference in Read Sample Size│ │ Garment Quantity ISO 2859-1 Table 1 & Ac / Re Pairs │ │ (e.g., 10,000 pcs) Level II -> Code "L" (200 pcs: 10/11)│ └─────────────────────────────────────────────────────────────────────────────┘
2. Defining an "Inspection Lot" in Apparel Manufacturing
Before any sampling table can be consulted, the QA professional must clearly define what constitutes an Inspection Lot.
Official Definition
Inspection Lot: A definite quantity of products, manufactured under essentially uniform conditions, submitted at one time for quality evaluation and acceptance testing.
Key Rules for Establishing Apparel Inspection Lots
- Homogeneity: All units in an inspection lot must be of the same garment style, made from the same fabric roll batches, and processed through the same sewing and finishing lines.
- Production Completeness: In standard apparel Pre-Shipment Inspection (PSI / FRI), 100% of the lot must be completely sewn, washed, pressed, and finished, and at least 80% must be fully packed in shipping master cartons with barcodes applied.
- Traceability: The lot must have clear identification markings, including Purchase Order (PO) number, style number, color breakdown, and destination country codes.
┌─────────────────────────────────────────────────────────────────────────────┐ │ WHAT CAN BE COMBINED AS A SINGLE LOT? │ │ │ │ VALID SINGLE LOT (Homogeneous) INVALID COMBINED LOT (Heterogeneous) │ │ ├── Style: Basic Crew-Neck Tee ├── Style A: Denim 5-Pocket Jeans │ │ ├── Fabric: 100% Cotton 180 GSM ├── Style B: Cotton Twill Chinos │ │ ├── Factory: Line 1 & Line 2 ├── Factory: Subcontractor Facility │ │ └── 3 Colorways (Same construction) └── 2 Unrelated Purchase Orders │ │ [STATUS: APPROVED FOR AUDIT] [STATUS: MUST BE SEPARATED] │ └─────────────────────────────────────────────────────────────────────────────┘
3. Step-by-Step Code Letter Determination (ISO 2859-1 Table 1)
The Sample Size Code Letter is an alphabetical code (A through R) that establishes the mathematical bridge between total lot size and sample quantity.
Table 1: Sample Size Code Letters Matrix
┌─────────────────────────┬──────────────┬──────────────┬──────────────┐ │ LOT SIZE RANGE (PCS) │ LEVEL I │ LEVEL II │ LEVEL III │ │ │ (Reduced) │ (Default) │ (Strict) │ ├─────────────────────────┼──────────────┼──────────────┼──────────────┤ │ 2 to 8 │ A │ A │ B │ │ 9 to 15 │ A │ B │ C │ │ 16 to 25 │ B │ C │ D │ │ 26 to 50 │ C │ D │ E │ │ 51 to 90 │ C │ E │ F │ │ 91 to 150 │ D │ F │ G │ │ 151 to 280 │ E │ G │ H │ │ 281 to 500 │ F │ H │ J │ │ 501 to 1,200 │ G │ J │ K │ │ 1,201 to 3,200 │ H │ K │ L │ │ 3,201 to 10,000 │ J │ L │ M │ │ 10,001 to 35,000 │ K │ M │ N │ │ 35,001 to 150,000 │ L │ N │ P │ │ 150,001 to 500,000 │ M │ P │ Q │ │ 500,001 and over │ N │ Q │ R │ └─────────────────────────┴──────────────┴──────────────┴──────────────┘
Common Apparel Lot Size Examples:
- Small Boutique Order (800 pcs): Range 501 to 1,200 → Code Letter J (Sample: 80 pcs).
- Medium Retail Order (2,500 pcs): Range 1,201 to 3,200 → Code Letter K (Sample: 125 pcs).
- Standard Export Order (8,000 pcs): Range 3,201 to 10,000 → Code Letter L (Sample: 200 pcs).
- Large Mass-Market Order (20,000 pcs): Range 10,001 to 35,000 → Code Letter M (Sample: 315 pcs).
- Mega Volume Order (50,000 pcs): Range 35,001 to 150,000 → Code Letter N (Sample: 500 pcs).
4. Single Sampling Plan Tables: Normal Inspection (Table 2-A)
Once the Code Letter is established, the auditor references Table 2-A to find the exact sample size and Accept/Reject (Ac/Re) thresholds.
┌─────────────────────────────────────────────────────────────────────────────┐ │ ISO 2859-1 TABLE 2-A: SINGLE SAMPLING PLANS (NORMAL) │ ├───────┬─────────┬──────────────┬──────────────┬──────────────┬──────────────┤ │ CODE │ SAMPLE │ AQL 1.0 │ AQL 1.5 │ AQL 2.5 │ AQL 4.0 │ │LETTER │ SIZE │ Ac / Re │ Ac / Re │(Major Defect)│(Minor Defect)│ ├───────┼─────────┼──────────────┼──────────────┼──────────────┼──────────────┤ │ A │ 2 │ ↓ (0/1) │ ↓ (0/1) │ ↓ (0/1) │ ↓ (0/1) │ │ B │ 3 │ ↓ (0/1) │ ↓ (0/1) │ ↓ (0/1) │ ↓ (0/1) │ │ C │ 5 │ ↓ (0/1) │ ↓ (0/1) │ ↓ (0/1) │ ↓ (0/1) │ │ D │ 8 │ ↓ (0/1) │ ↓ (0/1) │ ↓ (0/1) │ 0 / 1 │ │ E │ 13 │ ↓ (0/1) │ ↓ (0/1) │ 0 / 1 │ 1 / 2 │ │ F │ 20 │ ↓ (0/1) │ 0 / 1 │ 1 / 2 │ 2 / 3 │ │ G │ 32 │ 0 / 1 │ 1 / 2 │ 2 / 3 │ 3 / 4 │ │ H │ 50 │ 1 / 2 │ 2 / 3 │ 3 / 4 │ 5 / 6 │ │ J │ 80 │ 2 / 3 │ 3 / 4 │ 5 / 6 │ 7 / 8 │ │ K │ 125 │ 3 / 4 │ 5 / 6 │ 7 / 8 │ 10 / 11 │ │ L │ 200 │ 5 / 6 │ 7 / 8 │ 10 / 11 │ 14 / 15 │ │ M │ 315 │ 7 / 8 │ 10 / 11 │ 14 / 15 │ 21 / 22 │ │ N │ 500 │ 10 / 11 │ 14 / 15 │ 21 / 22 │ ↑ (21/22) │ └───────┴─────────┴──────────────┴──────────────┴──────────────┴──────────────┘
How to Interpret the Decision Rule:
- Acceptance Number (Ac): If the total number of defective garments found in the sample is less than or equal to Ac, the lot PASSES.
- Rejection Number (Re): If the total number of defective garments found in the sample is greater than or equal to Re, the lot FAILS.
- Re = Ac + 1: In single normal sampling, the rejection number is always strictly one unit higher than the acceptance number.
5. Master Carton Sampling Math: The Square Root Rule
An auditor cannot inspect sample garments without first opening master shipping cartons. To guarantee unbiased, representative sampling across the entire warehouse stack, industry standard SOP mandates the Square Root Rule:
extNumber of Cartons to Open = sqrt extTotal Master Cartons in Lot + 1
┌─────────────────────────────────────────────────────────────────────────────┐ │ CARTON SELECTION & DRAWING CALCULATION │ │ │ │ ORDER: 6,000 pcs packed 20 pcs/carton = 300 cartons │ │ FORMULA: √(300) + 1 = 17.3 + 1 ≈ 19 Cartons │ │ │ │ SAMPLE SIZE: Lot 6,000 at Level II = Code Letter "L" = 200 Garments │ │ PIECES PER CARTON: 200 pcs / 19 cartons ≈ 10 to 11 pcs per carton │ └─────────────────────────────────────────────────────────────────────────────┘
Warehouse Pallet Extraction Protocol:
Cartons must be pulled systematically from all dimensions of pallet stacks:
- Top Layer: 25% of selected cartons.
- Middle Layer: 50% of selected cartons.
- Bottom / Back Layer: 25% of selected cartons.
6. Multi-SKU Proportional Sample Allocation
When a purchase order contains multiple colorways and sizes, drawing samples evenly without considering the packing breakdown creates severe sampling bias.
Real Example: 10,000 Pcs Knit Polo Shirt Order
- Total Lot: 10,000 pcs (Code Letter L = 200 Sample Garments).
- Color Breakdown:
- Navy Blue: 5,000 pcs (50%)
- Heather Grey: 3,000 pcs (30%)
- Optic White: 2,000 pcs (20%)
┌─────────────────────────────────────────────────────────────────────────────┐ │ PROPORTIONAL SAMPLE ALLOCATION TABLE │ ├──────────────┬──────────────┬─────────────┬──────────────┬──────────────────┤ │ COLORWAY │ ORDER PCS │ % OF TOTAL │ SAMPLE PCS │ CARTONS OPENED │ ├──────────────┼──────────────┼─────────────┼──────────────┼──────────────────┤ │ Navy Blue │ 5,000 │ 50% │ 100 pcs │ 10 cartons │ │ Heather Grey │ 3,000 │ 30% │ 60 pcs │ 6 cartons │ │ Optic White │ 2,000 │ 20% │ 40 pcs │ 4 cartons │ ├──────────────┼──────────────┼─────────────┼──────────────┼──────────────────┤ │ TOTAL │ 10,000 │ 100% │ 200 pcs │ 20 cartons │ └──────────────┴──────────────┴─────────────┴──────────────┴──────────────────┘
7. Step-by-Step QA/QC Inspection Procedure (SOP)
┌─────────────────────────────────────────────────────────────────────────────┐ │ GARMENT FINAL INSPECTION WORKFLOW SOP │ │ │ │ [1. ARRIVAL & STAGING CHECK] │ │ └── Verify PO#, style, packing list, 100% finished, ≥80% packed. │ │ │ │ [2. CARTON COUNT & DRAWING] │ │ └── Apply √(Cartons)+1 rule. Physically pick cartons across all pallets. │ │ │ │ [3. PACKAGING & MARKING AUDIT] │ │ └── Check barcodes, shipping marks, polybag suffocation warnings. │ │ │ │ [4. WORKMANSHIP & DEFECT TALLY] │ │ └── Clockwise inspection under 1,000 lux D65 light. Tally defective units.│ │ │ │ [5. MEASUREMENT AUDIT (POM)] │ │ └── Measure 8–13 garments across size set against approved tech pack. │ │ │ │ [6. FINAL PASS/FAIL DISPOSITION] │ │ └── Compare defect counts against Ac/Re. Issue official Inspection Report.│ └─────────────────────────────────────────────────────────────────────────────┘
8. Summary & Key Takeaways
- AQL 2.5 is the global benchmark governing major defect acceptance in apparel export supply chains.
- ISO 2859-1 Table 1 maps total finished garment lot quantities to Sample Size Code Letters under General Inspection Level II.
- Table 2-A establishes the exact sample size and Accept/Reject threshold pairs for Critical (0.0), Major (2.5), and Minor (4.0) defects.
- Square Root Rule: Always open $sqrt{N} + 1 master cartons across all pallet layers to prevent sampling bias.
- Defective Unit Basis: Tally defective garments, not individual defect instances, against AQL threshold numbers.
1. Master Lot Size to Code Letter Mapping (General Inspection Levels I, II, III)
| Lot Size Range (Units) | General Level I (Reduced) | General Level II (Standard Default) | General Level III (Tightened/Strict) | Sample Size Level II (Pcs) |
|---|---|---|---|---|
| 2 to 8 | A | A | B | 2 pcs (Follow arrow ↓ to 13) |
| 9 to 15 | A | B | C | 3 pcs (Follow arrow ↓ to 13) |
| 16 to 25 | B | C | D | 5 pcs (Follow arrow ↓ to 13) |
| 26 to 50 | C | D | E | 8 pcs (Follow arrow ↓ to 13) |
| 51 to 90 | C | E | F | 13 pcs (Ac=0, Re=1) |
| 91 to 150 | D | F | G | 20 pcs (Ac=1, Re=2) |
| 151 to 280 | E | G | H | 32 pcs (Ac=2, Re=3) |
| 281 to 500 | F | H | J | 50 pcs (Ac=3, Re=4) |
| 501 to 1,200 | G | J | K | 80 pcs (Ac=5, Re=6) |
| 1,201 to 3,200 | H | K | L | 125 pcs (Ac=7, Re=8) |
| 3,201 to 10,000 | J | L | M | 200 pcs (Ac=10, Re=11) |
| 10,001 to 35,000 | K | M | N | 315 pcs (Ac=14, Re=15) |
| 35,001 to 150,000 | L | N | P | 500 pcs (Ac=21, Re=22) |
| 150,001 to 500,000 | M | P | Q | 800 pcs (Follow arrow ↑ to 500) |
| 500,001 and over | N | Q | R | 1,250 pcs (Follow arrow ↑ to 500) |
2. Standard Apparel AQL Accept / Reject Thresholds (Single Normal Sampling)
| Sample Size Code Letter | Sample Size (Garments) | Critical AQL 0.0 (Ac / Re) | Major AQL 1.5 (Ac / Re) | Major AQL 2.5 (Ac / Re) | Minor AQL 4.0 (Ac / Re) |
|---|---|---|---|---|---|
| Code E | 13 pcs | 0 / 1 | ↓ (0 / 1) | 0 / 1 | 1 / 2 |
| Code F | 20 pcs | 0 / 1 | 0 / 1 | 1 / 2 | 2 / 3 |
| Code G | 32 pcs | 0 / 1 | 1 / 2 | 2 / 3 | 3 / 4 |
| Code H | 50 pcs | 0 / 1 | 2 / 3 | 3 / 4 | 5 / 6 |
| Code J | 80 pcs | 0 / 1 | 3 / 4 | 5 / 6 | 7 / 8 |
| Code K | 125 pcs | 0 / 1 | 5 / 6 | 7 / 8 | 10 / 11 |
| Code L | 200 pcs | 0 / 1 | 7 / 8 | 10 / 11 | 14 / 15 |
| Code M | 315 pcs | 0 / 1 | 10 / 11 | 14 / 15 | 21 / 22 |
| Code N | 500 pcs | 0 / 1 | 14 / 15 | 21 / 22 | ↑ (21 / 22) |
3. Master Carton Selection Guide for Pre-Shipment Inspection (PSI)
| Total Cartons in Shipment Lot | Formula: √(Total Cartons) + 1 | Minimum Cartons to Open | Garments Sampled per Carton (Code L: 200 pcs) |
|---|---|---|---|
| 10 to 25 cartons | √(25) + 1 = 6 | 6 cartons | 33–34 pcs / carton |
| 26 to 50 cartons | √(50) + 1 = 8.1 | 8 cartons | 25 pcs / carton |
| 51 to 100 cartons | √(100) + 1 = 11 | 11 cartons | 18–19 pcs / carton |
| 101 to 200 cartons | √(200) + 1 = 15.1 | 15 cartons | 13–14 pcs / carton |
| 201 to 400 cartons | √(400) + 1 = 21 | 21 cartons | 9–10 pcs / carton |
| 401 to 700 cartons | √(700) + 1 = 27.5 | 28 cartons | 7–8 pcs / carton |
| 701 to 1,000 cartons | √(1,000) + 1 = 32.6 | 33 cartons | 6 pcs / carton |
| 1,001 to 2,000 cartons | √(2,000) + 1 = 45.7 | 46 cartons | 4–5 pcs / carton |
Practical Commercial Flow
An export quality audit was conducted at an RMG factory in Gazipur, Bangladesh for a 24,000-piece order of Men’s Long-Sleeve Oxford Shirts packed 24 pcs per carton (1,000 cartons total) across 3 colorways (White 12,000, Sky Blue 7,200, Navy 4,800) and 4 sizes (S, M, L, XL):
Factory Real-Life Scenario
Scenario: A woven bottoms factory in Ashulia presented a lot of 18,000 pcs Cargo Shorts for final inspection. The third-party auditor selected Code Letter M (315 pcs). During the audit, the inspector counted 17 Major defective garments (mainly skipped stitches on pocket flaps and skewed waistband topstitching) against an Accept limit of 14 (Reject = 15). The lot was failed.
Solution: The factory QA Manager halted shipment, instituted a 100% sorting protocol on all 18,000 garments, and deployed 30 skilled rework operators. Over 48 hours, 100% of pocket flaps were checked, 840 defective garments were repaired, pressed, and re-packed. The sewing supervisor re-calibrated the lockstitch thread tensions on Lines 4 and 6.
Impact: On re-inspection under Normal Level II Code M (315 pcs), only 4 Major defects and 8 Minor defects were found (well within Ac=14). The shipment was approved, preventing a buyer cancellation and a $62,000 air shipment debit note.
Commercial Officer Best Practice
When inspecting mixed solid-color cartons and pre-pack ratio cartons, always audit both the garment workmanship and the carton packing accuracy (assortment, polybag barcode, hangtag price sticker, silica gel placement) as part of the PSI protocol.
Top Beginner Mistakes to Avoid
- Assuming that AQL 2.5 means 2.5% of the total bulk shipment is guaranteed to be defective or allowed to fail.
- Counting multiple defects on one single garment as multiple defective units in the AQL sampling tally (artificially inflating rejection numbers).
- Selecting cartons only from the top or front layers of warehouse pallets (sampling bias).
- Failing to follow downward or upward arrows in ISO 2859-1 Table 2-A when inspecting small or specialized order quantities.
- Combining two completely different styles or fabrications into one combined inspection lot.
- Overlooking Critical safety defects (e.g., loose snaps, sharp pins) when Major defect numbers are within acceptable limits.
Important Commercial & Trade Terminology (6 Terms)
A statistical inspection procedure where the decision to accept or reject a lot is based on the results obtained from a single sample of predetermined size.
💡 Note: Most common sampling method in apparel pre-shipment audits.An alphabetical letter (A to R) assigned based on lot quantity and inspection level that determines the exact sample size to be inspected.
💡 Note: Found in ISO 2859-1 / ANSI ASQ Z1.4 Table 1.The maximum number of defective units found in the sample that still allows the lot to pass inspection.
💡 Note: If defective units <= Ac, lot passes.The minimum number of defective units found in the sample that requires the lot to be rejected.
💡 Note: In single normal sampling, Re is always equal to Ac + 1.An individual garment containing one or more defects of a specific category.
💡 Note: AQL sampling counts defective garments, not individual defect instances.The standard industry formula (√(Total Cartons) + 1) used to determine how many shipping cartons must be opened during a final inspection.
💡 Note: Ensures representative cross-section across the entire warehouse stack.Interview Preparation Questions & Answers (4 Questions)
Master these high-frequency interview questions for Commercial Officer, Merchandising, and Export Executive roles.
In ISO 2859-1 Table 1, find the lot size range "10,001 to 35,000". Looking across to the column for General Inspection Level II, the intersecting Sample Size Code Letter is "M". Turning to Table 2-A, Code Letter M corresponds to a sample size of 315 garments, with AQL 2.5 Major limits of Ac=14, Re=15 and AQL 4.0 Minor limits of Ac=21, Re=22.
Frequently Asked Questions (3 FAQs)
Clear, beginner-friendly answers to the most common questions about the Commercial Department.
Key Takeaways & Summary Points
Executive Lesson Summary
AQL 2.5 Sampling Plans under ISO 2859-1 provide a standardized, statistically rigorous framework for evaluating apparel shipments. By accurately determining lot sizes, locating Sample Size Code Letters, executing proportional carton pulling, and applying precise Accept/Reject limits across Critical, Major, and Minor defect classes, QA professionals protect brand reputation, minimize commercial disputes, and ensure consistent garment quality.
Lesson Downloads & SOP Templates
AQL 2.5 Sampling Calculator
Calculate sample sizes, code letters, and Ac/Re limits instantly.
Open Tool