AQL Calculator: Complete Guide to Acceptable Quality Limit Calculations

AQL Calculator – A practical walkthrough of ANSI/ASQ Z1.4 sampling plans — how to read the tables, calculate your sample size, and make accept/reject decisions with confidence.

1. What Is  AQL?

AQL stands for Acceptable Quality Limit — the maximum percentage of defective units that is still considered acceptable in a production batch. It is the cornerstone of statistical quality control used by manufacturers, importers, and quality auditors worldwide.

The most widely used AQL standard is ANSI/ASQ Z1.4, which defines sampling procedures for attribute inspection (pass/fail, conforming/nonconforming). Instead of checking every single unit in a shipment — which is time-consuming and expensive — you inspect a statistically determined sample and use the results to accept or reject the entire lot.

 

Key Concept AQL does NOT guarantee zero defects. It defines a statistically acceptable risk level. An AQL of 1.0% means you tolerate up to 1 defect per 100 units on average over many lots — but any individual lot could have more or fewer defects.

 

2. The Three Inputs to Every AQL Calculator Calculation

Every AQL sampling plan requires exactly three inputs. Get these right and the rest is just looking up the table.

 

2.1  Lot Size (N)

The total number of units in the batch being inspected. This is the starting point for determining your sample size.

📌 Example: Lot Size You receive a shipment of 3,500 phone cases. Your lot size N = 3,500.

 

2.2  Inspection Level

Inspection level controls how discriminating the sampling plan is — higher levels use larger samples. There are seven levels:

• General Level I — reduced scrutiny, smaller sample, higher consumer risk
• (Default) General Level II — the default for most industries and situations
• General Level III — increased scrutiny, larger sample, lower risk
• Special Levels S-1 through S-4 — very small samples for destructive or costly testing

 

Unless your industry or contract specifies otherwise, always use General Level II.

 

2.3  AQL Value

The AQL value defines your quality threshold. Common values and their typical use cases:

 

AQL Value	Typical Use	Strictness
0.065%	Critical safety components, medical devices	Extremely strict
0.10%	Aerospace, pharmaceutical	Very strict
0.65%	Electrical components, precision parts	Strict
1.0%	Consumer electronics, apparel hardware	Standard
1.5%	General merchandise, packaging	Moderate
2.5%	Non-critical consumer goods	Lenient
4.0%	Minor cosmetic defects	Very lenient

 

3. Step-by-Step AQL Calculator Calculation

The AQL Calculator calculation follows a fixed two-step process: first find your sample size code letter, then look up the accept/reject numbers.

 

Step 1 — Find the Sample Size Code Letter

Cross-reference your lot size with your inspection level in Table 1 below to get a single letter (A through R). This letter represents the sample size tier.

 

Table 1: Sample Size Code Letters (ANSI/ASQ Z1.4)

Lot Size	Level I	Level II (Default)	Level III
2 – 8	B	B	C
9 – 15	C	C	D
16 – 25	C	D	E
26 – 50	D	E	F
51 – 90	D	F	G
91 – 150	E	G	H
151 – 280	E	H	J
281 – 500	F	J	K
501 – 1,200	G	K	L
1,201 – 3,200	H	L	M
3,201 – 10,000	J	M	N

 

Step 2 — Find the Sample Size

Each code letter maps to a specific sample size. These are standardized across all AQL calculations:

 

A	B	C	D	E	F	G	H	J	K	L	M	N	P	Q	R
2	3	5	8	13	20	32	50	80	125	200	315	500	800	1250	2000

 

Step 3 — Find Accept (Ac) and Reject (Re) Numbers

Using your code letter and AQL value, look up the accept and reject numbers from the master table. The accept number (Ac) is the maximum defects allowed to pass the lot. The reject number (Re) is the minimum defects that triggers rejection.

 

Table 2: Accept / Reject Numbers by Code Letter and AQL (Normal Inspection)

Code	Sample	AQL 0.65%		AQL 1.0%		AQL 2.5%
Letter	Size	Ac	Re	Ac	Re	Ac	Re
F	20	0	1	0	1	1	2
G	32	0	1	1	2	1	2
H	50	1	2	1	2	2	3
J	80	1	2	2	3	3	4
K	125	2	3	3	4	5	6
L	200	3	4	5	6	7	8

 

4. The Accept/Reject Decision

Once you have your sample size, Ac, and Re values, the decision rule is straightforward:

 

Defects Found ≤ Ac  →  ACCEPT the lot The lot meets your quality standard — release it.

 

Defects Found ≥ Re  →  REJECT the lot The lot fails your quality standard — quarantine and review.

 

Note on Ac and Re You may notice that Re = Ac + 1 in most cases. This is intentional — there is no ambiguous middle ground. Every inspection result leads to either a clear accept or a clear reject decision.

 

5. Worked Example

📌 Example: Full Calculation Walkthrough Scenario: You are importing 2,500 units of USB cables. Inspection Level: General II (default) AQL: 1.0%   Step 1 — Find code letter: Lot size 2,500 falls in the range 1,201 – 3,200. At General Level II, the code letter is L.   Step 2 — Find sample size: Code letter L = 200 units to inspect.   Step 3 — Find Ac and Re: Code L, AQL 1.0%: Ac = 5, Re = 6.   Decision: You inspect 200 cables. If you find 5 or fewer defects → ACCEPT. If you find 6 or more defects → REJECT the entire shipment.

 

6. Inspection Types: Normal, Tightened, and Reduced

AQL plans have three severity levels that are switched based on recent inspection history:

 

Type	When to Use	Effect on Plan
Normal	Default — start here for all new suppliers	Standard sample size and Ac/Re values
Tightened	Switch when 2 of 5 consecutive lots are rejected	Stricter Ac — same sample, harder to pass
Reduced	Switch when 10 consecutive lots pass on Normal	Smaller sample size — reward for consistent quality

 

Switching Rules Summary Normal → Tightened: 2 rejections in last 5 lots. | Tightened → Normal: 5 consecutive lots accepted. | Normal → Reduced: 10 consecutive accepted + production steady + approved by QA. | Reduced → Normal: 1 rejection, or irregular production, or other conditions met.

 

7. Defect Classification

Most AQL programs use different AQL thresholds for different defect severities. The three standard categories are:

 

Category	Definition	Typical AQL	Examples
Critical	Hazardous or unsafe for the end user	0% (zero tolerance)	Electrical shock risk, toxic material, structural failure
Major	Likely to cause product failure or strong customer dissatisfaction	0.65% – 1.0%	Non-functional feature, wrong size, significant damage
Minor	Unlikely to affect function, slight appearance issue	1.5% – 4.0%	Small scratch, light stain, slight color variation

 

In practice, you would run three separate AQL inspections on the same sample — one for each defect category — and apply the appropriate accept/reject threshold to each.

 

8. Sample Percentage vs. Statistical Reliability

A common misconception is that a larger percentage sample means better protection. AQL tables are designed so that statistical reliability depends on the absolute sample size, not the percentage of the lot.

 

Lot Size	Sample Size (L-II, AQL 1.0%)	Sample %	Note
500	80	16.0%	High % due to small lot
3,200	200	6.25%
10,000	315	3.15%
35,000	500	1.43%
150,000	800	0.53%	Low % is statistically sufficient

 

This is why AQL is scalable: a 500-unit lot needs 80 samples (16%) while a 150,000-unit lot only needs 800 (0.53%) — yet both provide comparable statistical confidence for their respective quality thresholds.

 

9. Quick Reference Checklist

Use this checklist every time you run an AQL inspection:

 

1. Define your lot size (N) — count all units in the shipment or batch.
2. Choose your inspection level — General II unless specified otherwise.
3. Set your AQL value — agree with your buyer/supplier in advance.
4. Look up the code letter from Table 1.
5. Find your sample size from the code letter table.
6. Look up Ac and Re from Table 2.
7. Select a random sample — use random number tables or a digital randomizer.
8. Inspect each unit and record defects found.
9. Apply the decision rule: defects ≤ Ac = ACCEPT; defects ≥ Re = REJECT.
10. Document results and update your switching status for future inspections.

 

 

Use Our Free AQL Calculator Skip the manual table lookups — our AQL Calculator on this page does all of this automatically. Enter your lot size, inspection level, and AQL value, then click Calculate Sample Plan to get your sample size, accept number, and reject number instantly.