Calibration Guard Band Calculator
When measurement uncertainty is significant compared with the tolerance, tighten the acceptance limits (guard band) so that instruments close to the edge are not wrongly passed.
- Acceptance limit = tolerance − U = 0.25 − 0.08 = ± 0.17
Simple Guard Band
TUR = T ÷ U
T is the tolerance half width and U the expanded uncertainty (k = 2). This simple method (often called a 100 % guard band) keeps the false accept risk below about 2.5 %. Your quality procedure may specify a different guard band.
Worked example
Tolerance ± 0.25, uncertainty ± 0.08: the acceptance limit is ± 0.17. A measured error of 0.20 is inside tolerance but outside the acceptance limit, so the result is indeterminate.
Frequently Asked Questions
What is a guard band in calibration?
A margin subtracted from the tolerance to form tighter acceptance limits, accounting for measurement uncertainty.
When is guard banding needed?
When the TUR is low (for example below 4 : 1) or when a standard or customer requires a decision rule that accounts for uncertainty.
What is ISO/IEC 17025 decision rule?
Accredited labs must state how uncertainty is considered when declaring pass or fail; guard banding is one common decision rule.
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