International Standards and Conformity Assessment for all electrical, electronic and related technologies
Main title

SI Zone

International System of Units

The present situation in the IEC


current situation

Present situation in the IEC

IEC Technical Committee 25 was established in 1935. Its title was “Quantities and units, and their letter symbols”. Since its subsequent merger with TC 24 and as a committee with “horizontal” responsibilities (i.e. covering matters of a wide-ranging nature and applicable by many “vertical”, or product-oriented, committees), it is TC 25 that is at present in charge of all questions concerning the SI.

Scope of IEC TC 25

The scope of IEC TC 25 is to prepare International Standards on quantities and units to be used generally in electrical technology, and to review the use of quantities and units in IEC standards. These standards should, whenever possible, be based on the SI. Such standards may be related to definitions, names, letter symbols and their use, to the relations in which these quantities and units appear, and to the signs and symbols used with them.


Close co-operation is required with IEC TC 1: ”Terminology”, and technical liaison is maintained in particular with the following organizations:

BIPM - International Bureau of Weights and Measures

ISO/TC 12 - Quantities, units, symbols, conversion factors
ITU - International Telecommunication Union

OIML - International Organization of Legal Metrology

Methodology of TC 25

A broad and truly international consensus has to be reached on the fundamential concepts developed in the context of the IEC TC 25, so that the IEC member countries and other international organizations can be in the position to adopt them.


The following is then noteworthy:

  • out of more than 60 IEC member countries, 16 are P-members of TC 25 (i.e. participating members with voting rights);
  • requests for new concepts, including quantities and units and their letter symbols, may be submitted by any TC/SC or NC of the IEC;
  • committee drafts are prepared either by the project leader or a TC 25 Working Group in which experts from the relevant TCs/SCs and other organizations may participate if necessary;
  • NCs consult their experts in the field of TC 25 and of any other committee concerned in parallel, and submit their collated comments in a single document;
  • the resulting draft is circulated by IEC Central Office to all parties concerned, including other international organizations.


IEC 80000, Quantities and units


The harmonized IEC 80000, approved by both the IEC Standardization Management Board and the ISO Technical Management Board, consists of several parts (*) as listed below. The information in parentheses indicates the origin of the technical content of each part.

IEC 80000:


Part 1 General (IEC 60027-1 and IEC 60027-3, ISO 31-0)

Part 2 Mathematical signs and symbols to be used in the natural sciences and technology (IEC 60027-1, ISO 31-11)

Part 3 Space and time (ISO 31-1 and ISO 31-2)

Part 4 Mechanics (ISO 31-3)

Part 5 Thermodynamics (ISO 31-4)

Part 6 Electromagnetism ( IEC 60027-1, ISO 31-5)

Part 7 Light (ISO 31-6)

Part 8 Acoustics ( ISO 31-7)

Part 9 Physical chemistry and molecular physics (ISO 31-8)

Part 10 Atomic and nuclear physics (ISO 31-9 and ISO 31-10)

Part 11 Characteristic numbers (ISO 31-12)

Part 12 Solid state physics (ISO 31-13)

Part 13 Information science and technology (IEC 60027-2 and IEC 60027-3)

Part 14 Telebiometrics related to human physiology (this is a new project that is to replace IEC 60027-7)

It should be noted that the prefix before the number 80000 is either ISO or IEC, but not a double ISO/IEC prefix.


There are four systems of writing that bridge all linguistic barriers regardless of the alphabet used. These systems are:

  • the set of mathematical signs and symbols;
  • the SI;
  • the symbols for chemical elements; and
  • the way of writing notes for music.

The fundamental importance of ISO/IEC 80000 is obvious because the first three systems will be given in this standard. It is only music that will be outside the scope of the future ISO/IEC 80000.


* Copies of IEC International Standards are available from the IEC Webstore or from IEC National Committees and approved sales outlets.