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AQL Sampling Calculator

Find the sample size and acceptance/rejection numbers for a lot inspection.

Code letter H, sample size 50

Ac 3 / Re 4

Sample size 50 at code letter H

Consistent with, but not a literal reproduction of, the published ANSI/ASQ Z1.4 table — verify against the official standard for regulatory or contractual use.

How AQL sampling works

Acceptance sampling inspects a smaller sample from a lot rather than every unit, and uses the sample's defect count to decide whether to accept or reject the whole lot — the standard quality-control method for incoming or outgoing inspection.

Enter the lot size and your Acceptable Quality Limit (AQL) — the maximum percent defective considered acceptable as a process average. The calculator returns the sample size (from the standard General Inspection Level II lot-size table) and the acceptance/rejection numbers.

The acceptance number is derived directly from the Poisson approximation to the binomial distribution — the same statistical basis the published ANSI/ASQ Z1.4 (ISO 2859-1) standard's tables are built on — rather than copied from a specific published table. For regulatory or contractual inspection, verify against the official published standard.

The formula

n from the Z1.4 Level II lot-size table. Ac = largest c where P(defects ≤ c | n, AQL) ≥ 95%.

Sample size comes from the standard lot-size-to-code-letter table. The acceptance number is the largest defect count where the Poisson probability of observing that many or fewer defects — given the sample size and the AQL as the defect rate — is at least 95%.

Worked example

A lot of 500 units, AQL of 2.5%

Lot size 500 maps to code letter H with a sample size of 50. At AQL 2.5%, the acceptance number works out to 3 (accept if 3 or fewer defects are found; reject at 4 or more).

Frequently asked questions

What inspection level does this use?

General Inspection Level II — the standard default used for most inspections unless your quality plan specifies otherwise.

Is this the exact official Z1.4 table?

The sample-size table is the standard published lot-size mapping. The acceptance number is computed from first principles (the Poisson approximation) rather than copied from the table, and is consistent with it — for contractual or regulatory submissions, verify against the official published ANSI/ASQ Z1.4 / ISO 2859-1 standard directly.

What does a lower AQL mean?

A lower AQL is a stricter quality requirement — it means a smaller number of defects will cause the lot to be rejected, for the same sample size.

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