How Should Calibration Intervals Be Determined: Why One Year Is Not a Universal Rule

This working paper explains how calibration intervals should be determined and why a fixed one-year interval is not suitable for every measuring instrument. It presents a practical, risk-based approach based on intended use, measurement criticality, equipment stability, operating conditions, maintenance history and previous calibration results. It also discusses the use of intermediate checks, control charts and documented reviews when extending or shortening an interval. Examples from medical metrology show how the consequences of measurement error can influence the decision. The paper also clarifies the responsibilities of calibration laboratories and equipment users under ISO/IEC 17025. The aim is to support laboratories, healthcare institutions, quality managers and equipment owners in developing calibration programmes that are technically justified, clearly documented and proportionate to risk.