Operating Characteristic curve
Calculator Library / Inspection
AQL Sampling Calculator
Estimate acceptance and rejection numbers from lot size and AQL, then see the full Operating Characteristic curve so incoming quality and receiving teams understand exactly what each plan will and will not catch.
Everything updates live. A tighter AQL or higher inspection level pulls a bigger sample and drops the reject threshold. Confirm the final plan against the formal standard your organization releases against.
Instructions
How to use this app
- Enter the lot size (N) to be inspected. The tool maps it to a sample-size code letter.
- Pick the inspection level and AQL target. General II is the default; go to General III for more discrimination or General I to inspect fewer parts.
- Optionally enter the expected incoming defect rate to mark where your supplier actually sits on the curve.
- Read the Operating Characteristic curve: it plots the probability the plan accepts a lot against that lot’s true defect rate. The steeper it is, the sharper the plan separates good lots from bad.
- Use the producer risk, RQL, and AOQL read-outs to judge risk, then align the released plan to the formal sampling standard your organization or customer requires.
What This AQL Calculator Helps You Decide
This calculator helps receiving, supplier quality, and incoming inspection teams estimate how many units to inspect and where the acceptance and rejection threshold should sit for a given lot size and AQL target.
It is intended as a practical planning tool. Final released plans should still be checked against the exact sampling standard and company policy in force.
Core Logic
| Step | Meaning |
|---|---|
| Lot size to code letter | Maps the lot into a standard sample-size band. |
| Code letter to sample size | Converts the band into an inspection quantity. |
| AQL to accept or reject threshold | Defines how many defects are allowed before the lot is rejected. |
| Operating Characteristic curve | Shows the probability of acceptance across every possible incoming defect rate, revealing producer and consumer risk. |
Worked Example
A lot of 2,500 pieces at General II with an AQL of 1.00% maps to code letter K in this planning model. That produces a sample size of 125 pieces with a small acceptance number and a reject number one defect higher. The OC curve then shows that a lot running right at the 1% AQL is still accepted about 96–98% of the time, while a lot running near 5% defective — the RQL — is accepted only about one time in ten.
How to Interpret the Results
- Larger lot sizes: generally drive the code letter and sample size upward, steepening the curve.
- Tighter AQL: lowers the number of defects tolerated before rejection and shifts the whole curve left.
- Expected defect rate above AQL: reduces the chance the lot will pass — watch where the amber marker lands on the curve.
- AOQL: the worst average outgoing quality the plan allows once rejected lots are screened and corrected.
- Planning estimate only: use the result to prepare, then confirm against the official standard you follow.
AQL Sampling Frequently Asked Questions
What does AQL mean?
AQL means Acceptable Quality Level, the defect level used to design a sampling plan.
Does AQL guarantee that every accepted lot is good?
No. It is a probabilistic sampling approach, not a guarantee that every non-sampled unit is conforming.
Why does the calculator show accept and reject numbers?
Those numbers define the decision rule for the inspected sample. At or below the accept number the lot passes, and at or above the reject number it fails.
Can I use this instead of my company's official standard?
No. Use it for planning and education, then verify the final plan against the standard and customer requirements your organization uses.
When should AQL plans be tightened?
Tighten them when supplier performance degrades, escapes increase, or the item has higher quality, safety, or customer risk.
Related Templates and Guides
Read the AQL Sampling Guide
Use the guide for the inspection-level logic, switching rules, OC curve interpretation, and supplier-quality context behind this calculator.
Watch the AQL Sampling Video
Review the companion SixSigmaKaizen video lesson before building or explaining an incoming inspection plan.
Download the Six Sigma Calculator Suite
Use the workbook when inspection planning needs to connect to defect and capability analysis.
Read the Quality Standards Guide
Use the guide when inspection plans need to align with broader quality-system and customer requirements.