AQL (Acceptance Quality Limit) is the worst tolerable process average when a continuing series of lots is submitted for acceptance sampling. It is defined in ISO 2859-1 and represents the maximum percent defective considered acceptable as a process average.

How do I choose the right AQL value?

AQL values depend on the criticality of defects. Critical defects typically use AQL 0 (100% inspection). Major defects often use AQL 1.0 or 2.5. Minor defects may use AQL 4.0 or 6.5. The choice depends on the product and customer requirements.

What are inspection levels I, II, and III?

General inspection level II is the default. Level I uses smaller samples (less discrimination) and is used when less inspection is needed. Level III uses larger samples (more discrimination) for critical products requiring higher confidence. Inspection levels only select the sample size code letter — they are a separate concept from normal, tightened and reduced inspection, which are severities changed by the switching rules of ISO 2859-1.

What do Ac and Re numbers mean?

Ac (Accept number) is the maximum number of defectives allowed to accept the lot. Re (Reject number) is the minimum number of defectives that requires rejection. If defectives found ≤ Ac, accept the lot. If defectives ≥ Re, reject the lot.

What is the difference between normal, tightened, and reduced inspection?

Normal inspection is the default. If 2 out of 5 consecutive lots are rejected, switch to tightened inspection (lower Ac numbers). After 5 consecutive accepted lots under tightened, return to normal. Reduced inspection (smaller samples) is allowed after 10 consecutive lots accepted under normal with good quality.

Free Engineering Tool

AQL Sampling Table

Look up ISO 2859-1 sampling plans. Enter lot size, inspection level, and AQL value to get sample size, accept number (Ac), and reject number (Re).

This calculator was developed and verified by the Vibromera engineering team, makers of the Balanset portable balancing instruments.

ISO 2859-1ANSI/ASQ Z1.4Normal Inspection

Sampling Plan

Sample Size

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Accept Number (Ac)

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Reject Number (Re)

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Sample Code Letter

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AQL Sampling Process

1. Determine lot size → 2. Select inspection level (default: II) → 3. Find sample size code letter → 4. Look up Ac/Re numbers for your AQL value → 5. Inspect the sample → 6. If defectives ≤ Ac → Accept lot. If defectives ≥ Re → Reject lot.

Rule of thumb: AQL 2.5 with Level II is the most commonly used combination in manufacturing. It provides a good balance between inspection cost and quality assurance.

Levels vs. severity: The inspection level (S-1…S-4, I, II, III) only selects the sample size code letter. Normal, tightened and reduced inspection are a separate concept — inspection severities applied through the switching rules of ISO 2859-1 (clause 9) based on recent lot acceptance history. This calculator gives single sampling plans for normal inspection (Table 2-A); the inspection level stays unchanged when severity switches.

Sample Size Code Letters (Level II)

Lot Size	Code	Sample Size
2–8	A	2
9–15	B	3
16–25	C	5
26–50	D	8
51–90	E	13
91–150	F	20
151–280	G	32
281–500	H	50
501–1200	J	80
1201–3200	K	125
3201–10000	L	200
10001–35000	M	315
35001–150000	N	500
150001–500000	P	800
500001+	Q	1250

AQL (Acceptance Quality Limit) is the worst tolerable process average when lots are submitted for acceptance sampling. It represents the maximum percent defective considered acceptable. Defined in ISO 2859-1.

Critical defects: AQL 0 (100% inspection). Major defects: AQL 1.0 or 2.5. Minor defects: AQL 4.0 or 6.5. The most common general-purpose AQL is 2.5.

Level II is the default general inspection level. Level I = smaller samples (less discrimination). Level III = larger samples (more confidence). Special levels S-1 to S-4 use very small samples for destructive testing.

Inspection levels only select the sample size code letter. They are separate from normal / tightened / reduced inspection — those are severities applied via the switching rules, and the chosen level stays unchanged when severity switches.

Ac = maximum defectives to accept the lot. Re = minimum defectives to reject. If found ≤ Ac → Accept. If found ≥ Re → Reject.

Normal: default. Tightened: switch after 2 of 5 lots rejected (lower Ac). Reduced: after 10 consecutive accepted lots (smaller samples). ISO 2859-1 defines switching rules.

The procedures described on this page were developed and field-tested by the Vibromera team using the Balanset-1A, a portable dual-channel balancer and vibration analyzer for on-site rotor balancing.