This international arrangement was established to enable the mutual recognition of national measurement standards and of calibration and measurement certificates issued by national metrology institutes (NMIs) This arrangement was developed in response to a growing need for an open, transparent and comprehensive scheme to provide users with reliable quantitative information on the comparability of national metrology services and to provide the technical basis for wider agreements negotiated for international trade, commerce and regulatory affairs.

In October 1999, the Directors of 38 National Metrology Institutes (NMIs) and 2 International Organisations signed the Comité International des Poids et Mesures (CIPM) Mutual Recognition Arrangement (MRA), an historic agreement for the wider acceptance of National Measurement Standards and of Calibration and Measurement Certificates issued by their institutes. NPL negotiated the CIPM MRA on behalf of the UK and is the UK's signatory. By August 2007 the CIPM MRA had now been signed by the representatives of 67 institutes – from 45 Member States, 20 Associates of the CGPM, and 2 international organisations – and covers a further 119 institutes designated by the signatory bodies. All four UK NMS laboratories participate in the CIPM MRA.

The objectives of the CIPM MRA are:

• to establish the degree of equivalence of national measurement standards maintained by NMIs;
• to provide for the mutual recognition of calibration and measurement certificates issued by NMIs;
• thereby to provide governments and other parties with a secure technical foundation for wider agreements related to international trade, commerce and regulatory affairs.

Under the CIPM MRA participating NMIs and Designated Institutes are required to:

• Take part in appropriate comparisons
• Declare and subject their CMCs for extensive peer review
• Implement and demonstrate an appropriate quality system

One of the main aspects of the CIPM MRA is the BIPM Key Comparison Database (KCDB), which is divided in 4 appendices:

Appendix A: List of the CIPM MRA signatories
Appendix B: Results of CIPM and Regional Metrology Organisation key and supplementary comparisons
Appendix C: Calibration and Measurement Capabilities (CMCs)
Appendix D: List of key and supplementary comparisons

The Calibration and Measurement Capabilities (CMCs) listed in Appendix C give a description of the services offered by a National Measurement Institute that are covered by the CIPM MRA, together with their associated uncertainties.

View the CIPM website to see more details about the KCDB

The KCDB is maintained and regularly updated by the BIPM as new CMCs are submitted/revised and approved.

View the CIPM website for statistics and the latest progress on the MRA and KDBC

As a result of the CIPM MRA, the requirement for traceability to a particular NMI can be replaced with traceability to the SI provided by any NMI (or Designated Institute) that is a signatory to the CIPM MRA and which has relevant CMCs listed in the KCDB, whose listed scope and uncertainties meet the user’s requirements. In addition the KCDB provides measurement end users with a mechanism to compare directly the CMCs of the National Measurement Institutes.