A reliable assessment of measurement uncertainty in calibration services is an essential qualitative requirement for any calibration laboratory. While general guidance on the estimation of measurement uncertainty was published by the International Standards Organization (ISO) in 1995, that guidance lacked a specific focus on dosimetry calibration laboratories. This publication is devoted to the implementation of a common procedure of uncertainty assessment across all Secondary Standards Dosimetry Laboratories (SSDLs). It includes practical examples of the assessment of measurement uncertainty for air kerma and absorbed dose to water calibration for radiotherapy dosimetry. Implementation of the procedures recommended in this publication will ensure compliance with the ISO requirements and facilitate the review of comparison results in proficiency testing or audits.