Guard band & decision rule calculator
Enter a tolerance and your measurement uncertainty to get your test uncertainty ratio (TUR), guard-banded acceptance limits, and a ready-to-adapt ISO/IEC 17025 decision-rule statement. Everything runs in your browser.
Your numbers
The specification limit you judge against, as a half-width (±).
Your measurement or reference uncertainty at 95% (k = 2), same units as the tolerance.
A reading as a deviation from nominal (can be negative), same units. Compares the value against the limits; it does not compute false-accept probability.
Result
Decision rules, in plain terms
What a decision rule is
A decision rule is how you turn a measurement and its uncertainty into a Pass or Fail. ISO/IEC 17025:2017 asks laboratories to do this deliberately: when you report a statement of conformity, clause 7.1.3 says you record the decision rule you will use, and clause 7.8.6 says it is documented and applied. The point is that two labs measuring the same part should not quietly disagree on what counts as a pass.
Test uncertainty ratio (TUR)
The TUR is the tolerance divided by the expanded measurement uncertainty (k = 2). A ratio of 4:1 has been the accepted rule of thumb for decades: the measurement is good enough that you can accept anywhere inside the tolerance without much risk of being fooled by uncertainty. As the ratio falls toward 1:1, the uncertainty starts to eat into the tolerance, and a simple pass or fail at the limit becomes a coin toss.
Guard banding
A guard band moves the acceptance limit inward, away from the tolerance, by some amount w. A common binary choice (ILAC-G8) is w = U: accept only if the reading is inside the tolerance by at least one expanded uncertainty. That lowers the chance of accepting an item that is truly out of tolerance (a false accept, or consumer risk), at the cost of rejecting some genuinely good items near the edge (producer risk). Guard banding is how labs keep reporting conformity honestly when the TUR is below 4:1.
This calculator stops at transparent, widely used methods. A full probability-of-false-accept analysis under ANSI/NCSL Z540.3 needs an assumption about how your process is distributed, which is a judgment your laboratory makes, so it is left out rather than guessed.
Common questions
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What is a test uncertainty ratio (TUR)?
The TUR compares the tolerance you are checking against to the uncertainty of the measurement used to check it: the tolerance divided by the expanded measurement uncertainty at k = 2. A TUR of 4:1 means the tolerance is four times the measurement uncertainty. Higher is better, and 4:1 is a long-standing rule of thumb.
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What TUR do I need for ISO/IEC 17025?
ISO/IEC 17025 does not mandate a fixed ratio. The widely used guideline, inherited from earlier military and quality standards, is 4:1. Below that, laboratories commonly apply a guard band so false-accept risk stays controlled. What an assessor looks for is that you have defined and applied a documented decision rule.
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What is guard banding?
Guard banding tightens your acceptance limit to account for measurement uncertainty. Instead of accepting up to the full tolerance, you accept only up to the tolerance minus a guard band. It lowers the chance of passing an item that is actually out of tolerance (a false accept), at the cost of rejecting some good items near the limit.
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Does ISO/IEC 17025 require a decision rule?
Yes. When a statement of conformity is reported, clause 7.1.3 requires the laboratory to record the decision rule it will use, and clause 7.8.6 requires that the decision rule be documented and applied. This tool helps you choose one and draft the statement; the policy itself is owned by your laboratory.
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What is the difference between simple and guarded acceptance?
Simple acceptance (shared risk) accepts any result within the tolerance and does not subtract uncertainty; it is common when the TUR is high. Guarded acceptance subtracts a guard band from the tolerance, so a pass is claimed only inside the tighter acceptance limit, which reduces the risk of accepting a nonconforming item.
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How are asymmetric tolerances handled?
When a specification is wider on one side than the other (for example +0.2 / −0.1 from nominal), enter each half-width separately. The guard band is applied inward on each side independently, so the acceptance limit on each side is that side’s tolerance minus the guard band. The binding test uncertainty ratio is computed from the smaller (tighter) side, since that side governs whether the process is capable. If one side’s acceptance limit falls to zero or below, that side cannot claim a guarded pass while the other side may still be capable.
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Is this a substitute for ANSI/NCSL Z540.3 risk analysis?
No. It uses transparent, common methods (TUR and guard-banded acceptance in the style of ILAC-G8) to help you reason about a decision rule. A formal probability-of-false-accept analysis under ANSI/NCSL Z540.3 depends on your process distribution and remains your laboratory’s responsibility.
This calculator is an aid for reasoning about decision rules and guard bands. It does not set your laboratory’s risk policy or replace ISO/IEC 17025, ILAC-G8, or ANSI/NCSL Z540.3. Review any decision-rule statement against your own quality system before you use it.
Keep the decision rule with the record
Axiospec keeps every calibration, uncertainty, and certificate in one tamper-evident ledger built for ISO/IEC 17025. See it on real calibration data, no signup.