International comparisons take many forms. Some are carried out by the BIPM; others are organized by a Consultative Committee and may or may not include the BIPM; many more are organized and carried out, usually as part of the activities of a regional metrology organization, by the NMIs and other institutes listed in Appendix A of the CIPM MRA. Most BIPM comparisons are among the key comparisons chosen by the Consultative Committees to establish the degree of equivalence between national measurement standards.

For the comparisons carried out by the BIPM, the following procedures are used:

national standards are brought to the BIPM and compared with the BIPM standards, either individually or as a group, with similar standards from other national metrology institutes. The national standards may be either primary or secondary standards.

BIPM travelling primary standards are taken to a national metrology institute for comparison with those of that institute or, as increasingly happens, with standards from a group of national institutes from the same geographical region.

well-characterized BIPM travelling secondary standards are sent to a national metrology institute for comparison with the national standards of that institute.

This section lists ongoing BIPM key comparisons, mainly of the first type.