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Guard band and decision rule calculator

Turn a tolerance and an expanded uncertainty into acceptance limits under ILAC-G8 rules, and see the false-accept and false-reject risks.

Enter the tolerance as the specification limits (for an error tolerance of ±0.5 °C use −0.5 and 0.5, and evaluate the measured error). For a reading tolerance such as 2–8 °C, enter 2 and 8 and evaluate the reading.

Acceptance zone

Guard band w
0.120°C
1 × U
σ = U / k
0.0600°C
standard uncertainty
Accept from
-0.380°C
tolerance -0.5 + w
Accept to
0.380°C
tolerance 0.5 − w
PFA at accept limit
2.28 %
false accept, value on the line
PFR at tolerance limit
97.7 %
false reject, true value on the line
-0.700.72

red: tolerance · green: acceptance zone · blue: measured value ± U

Verdict for the measured value

Non-binary (ILAC-G8)
Conditional pass
Binary statement
Fail
pass only inside the acceptance zone
P(conformance)
95.2 %
true value within tolerance
Measured ± U
0.40 ± 0.12°C

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How the calculation works

  1. 01Guard band w = g × U, where g = 1 is the ILAC-G8 default (w = U) and g = 0 is simple acceptance.
  2. 02Acceptance limits: A_L = T_L + w, A_U = T_U − w.
  3. 03Risk model: measurement error ~ Normal(0, σ) with σ = U / k. Probability of false acceptance for a value on the acceptance limit = 1 − Φ(w/σ).
  4. 04Probability of conformance for a measured value x: Φ((T_U − x)/σ) − Φ((T_L − x)/σ) (JCGM 106 'specific risk').

Limitations

  • The normal error model is the conventional assumption; it is wrong when a single rectangular component dominates.
  • The tool evaluates one value against one tolerance; it does not decide which rule your customer agreed to.

Frequently asked questions

What guard band factor should I use?
ILAC-G8's common choice is w = U (factor 1), giving about 2.3 % probability of false acceptance at the acceptance limit with k = 2. Regulated customers often require it; simple acceptance (factor 0) shares the risk 50/50 at the limit.
What is a conditional pass?
A non-binary statement (ILAC-G8): the measured value is within tolerance but inside the guard band, so the true value may be outside. It is reported as conditional so the customer can decide.
Does guard banding change the measured value?
No. It changes only the conformity statement; the value and its uncertainty are reported unchanged.

Need the reasoning, not just the number?

Ask ValiTrac AI and see the standards evidence and the engine calculation behind the answer.

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Read the theory

Uncertainty

What is a decision rule and why every certificate needs one

A decision rule states how measurement uncertainty is accounted for when declaring pass or fail against a tolerance. Simple acceptance, guard banding and non-binary statements are the common choices.

2026-09-13 · 2 min read
Uncertainty

Guard banding explained with a worked example

A guard band narrows the acceptance zone by a multiple of the uncertainty so that a 'pass' has a controlled probability of being wrong. Here is the arithmetic.

2026-09-13 · 2 min read
Calibration

Tolerance versus uncertainty

Tolerance is what you require of the instrument; uncertainty is how well the calibration could measure it. A conformity statement needs both.

2026-09-13 · 2 min read
ISO/IEC 17025

Reporting results and statements of conformity (clause 7.8)

A certificate must carry defined identification, method, traceability and uncertainty information — and any pass/fail statement must name the decision rule.

2026-09-13 · 2 min read

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