The condition where an instrument's as-found calibration readings exceed the specified tolerance limits, indicating it was not performing within acceptable accuracy while in service.
An out-of-tolerance (OOT) finding during calibration means that the instrument's measurements were inaccurate beyond acceptable limits while it was in use. This is a significant quality event because it raises questions about the validity of all measurements made with the instrument since its last known good calibration. The severity of an OOT condition depends on how far outside tolerance the readings are and how critical the measurements made with the instrument were.
When an OOT condition is found, a formal investigation process should be triggered. This typically includes determining the magnitude and direction of the error, identifying all products, processes, or test results that were measured with the instrument during the affected period, assessing whether the OOT condition could have caused any of those measurements to be wrong enough to matter, and documenting the investigation findings and any corrective actions taken.
For calibration management, OOT tracking is a key performance indicator. The OOT rate (percentage of calibrations that find instruments out of tolerance) is used to evaluate the effectiveness of calibration intervals — a high OOT rate suggests intervals are too long, while a very low rate may indicate intervals could be safely extended. Most organizations target an OOT rate of 2-5% as a balance between measurement reliability and calibration efficiency. Calibration management software should automate OOT notifications, track investigations, and support trend analysis of OOT rates by instrument type, manufacturer, and usage pattern.
In an aerospace calibration lab, a Fluke 5520A multifunction calibrator used for testing avionics multimeters shows as-found readings of 10.025V when sourcing 10.000V, exceeding the ±0.002V tolerance limit. This OOT condition means flight-critical voltage measurements were potentially inaccurate during the aircraft's service period, requiring immediate investigation of any parts tested with this calibrator. In a medical device manufacturer's lab, a pressure calibrator supporting ventilator testing reads 101.8 kPa when the reference standard indicates 100.0 kPa, violating the ±0.5 kPa specification. This OOT finding triggers mandatory evaluation of all ventilators calibrated since the last successful verification, as patient safety depends on accurate pressure delivery. Getting OOT handling wrong causes severe problems: failing to document as-found readings before adjustment masks the true measurement uncertainty that existed during service. FDA auditors frequently cite manufacturers for inadequate OOT investigations, particularly when corrective actions don't address root causes. In AS9100 audits, aerospace suppliers face major nonconformances when OOT instruments aren't properly segregated and their impact on delivered products isn't assessed. Without proper OOT protocols, labs cannot demonstrate measurement traceability integrity or prove that previously released products met specifications.
ISO/IEC 17025:2017 addresses OOT in sections 7.8.7.1 and 7.8.7.2, requiring laboratories to take planned action when equipment is found to be outside acceptable limits and to evaluate the validity of previous results. AS9100D section 7.1.5.2.1 mandates investigation when measuring equipment is found unfit for use, with assessment of product already delivered. ISO 13485:2016 section 7.6 requires medical device manufacturers to investigate the impact of nonconforming measurement equipment on product conformity. ANSI/NCSL Z540.3-2006 section 10.2.4 specifically defines OOT procedures, requiring documentation of as-found conditions and evaluation of measurements made while out of tolerance. IATF 16949:2016 section 7.1.5.2.1 requires automotive suppliers to assess the validity of previous measurement results when equipment doesn't conform to requirements. Auditors specifically examine OOT investigation records, looking for: proper documentation of as-found readings, timely corrective actions, assessment of affected products, and evidence that measurement uncertainty was considered during the OOT period. ILAC P14:2013 emphasizes that accredited laboratories must have documented procedures for handling OOT equipment and demonstrate that measurement validity is maintained.
CalibrationOS automatically captures as-found readings before any adjustments through its Calibration Execution module, creating an immutable record that prevents accidental data loss. When measurements exceed tolerance limits, the system immediately flags OOT conditions and triggers workflow notifications to quality managers. The OOT Investigation module guides technicians through root cause analysis, linking affected instruments to all items calibrated during the suspect period using automated traceability reports. Certificate generation includes clear as-found/as-left statements with uncertainty calculations that account for OOT periods. The Audit Trail feature provides complete documentation of OOT events, corrective actions, and impact assessments that auditors require. Dashboard analytics identify trending toward OOT conditions, enabling preventive actions. During compliance audits, CalibrationOS generates comprehensive OOT summary reports showing investigation completeness, affected product lists, and corrective action effectiveness. The system's measurement uncertainty propagation automatically recalculates uncertainties for products tested during OOT periods, supporting regulatory requirements for measurement validity assessment.
An OOT finding triggers an investigation to assess the impact on measurements made since the last calibration. This includes identifying affected products/processes, evaluating the magnitude of error, and documenting corrective actions.
Most organizations target an OOT rate of 2-5%. A rate above 5% suggests calibration intervals are too long. A rate near 0% may indicate over-calibration and opportunities to extend intervals and reduce costs.
This article is licensed CC BY-SA 4.0. Share, adapt, and reuse with attribution to calibrationos.com/glossary/out-of-tolerance.
Industry benchmarks, best practices, and calibration tips — delivered to your inbox.
ILAC G24 staircase method — input pass/fail history and get a data-driven interval recommendation.
Open the Calculator