SI: current work

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

Present situation in the IEC


Present situation in the IEC

Methodology of TC 25

A broad and truly international consensus on the fundamental concepts developed by IEC TC 25 is the prerequisite for IEC Member countries and international organizations to be able to adopt them.


In this context, the following is noteworthy:

  • 16 IEC Member countries 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 (IEC Member);
  • Committee drafts are prepared either by the project leader or a TC 25 Working Group in which experts from the relevant TCs/SCs and from other organizations can participate;
  • NCs submit the collated comments from national experts in the field of TC 25 and of any other relevant committee in parallel, in a single document;
  • TC 25 drafts for new standards are circulated widely,  including to other international organizations.

IEC 80000: Quantities and units

The harmonized IEC 80000 series, 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. The prefix before each part is either IEC or ISO, the 80000 series never has a double logo.

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


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 International System of Units (SI);
  • the symbols for chemical elements; and
  • the way of writing notes for music.

Three of these four systems of writings are integral to the IEC and ISO 80000 series. Only music is outside of its scope.

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