Search Legislation

Council Directive of 20 December 1979 on the approximation of the laws of the Member States relating to units of measurement and on the repeal of Directive 71/354/EEC (80/181/EEC)

 Help about what version

What Version

  • Latest available (Revised)
  • Original (As adopted by EU)
 Help about advanced features

Advanced Features


This is a legislation item that originated from the EU

After exit day there will be three versions of this legislation to consult for different purposes. The version is the version that applies in the UK. The EU Version currently on EUR-lex is the version that currently applies in the EU i.e you may need this if you operate a business in the EU.

The web archive version is the official version of this legislation item as it stood on exit day before being published to and any subsequent UK changes and effects applied. The web archive also captured associated case law and other language formats from EUR-Lex.

Changes over time for: Division 1.

 Help about opening options


EU Directives are being published on this site to aid cross referencing from UK legislation. After exit day no further amendments will be applied to this version.


1.1. SI base units U.K.+E.U.
Quantity Unit
Name Symbol
Length metre m
Mass kilogram kg
Time second s
Electric current ampere A
Thermodynamic temperature kelvin K
Amount of substance mole mol
Luminous intensity candela cd
Definitions of SI base units: U.K.+E.U.
[F1Unit of length U.K.+E.U.

A metre is the length of the path travelled in a vacuum by light during 1/ 299 792 458 seconds.

(Seventeenth CGPM (1983), Resolution 1).]

Unit of mass U.K.+E.U.

The kilogram is the unit of mass; it is equal to the mass of the international prototype of the kilogram.

(Third CGPM (1901), page 70 of the conference report).

Unit of time U.K.+E.U.

The second is the duration of 9 192 631 770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the caesium-133 atom.

(Thirteenth CGPM (1967), resolution 1).

Unit of electric current U.K.+E.U.

The ampere is that constant current, which if maintained in two straight parallel conductors of infinite length, of negligible circular cross-section, and placed one metre apart in vacuum, would produce between those conductors a force equal to 2 × 10 -7 newton per metre of length.

(CIPM (1946), resolution 2, approved by the ninth CGPM (1948)).

[F2Unit of thermodynamic temperature U.K.+E.U.

The kelvin, unit of thermodynamic temperature, is the fraction 1/273,16 of the thermodynamic temperature of the triple point of water.

This definition refers to water having the isotopic composition defined by the following amount-of-substance ratios: 0,00015576 mole of  2 H per mole of  1 H, 0,0003799 mole of 17 O per mole of 16 O and 0,0020052 mole of 18 O per mole of  16 O.

(Thirteenth CGPM (1967), resolution 4 and Twenty-third CGPM (2007), resolution 10)]

Unit of amount of substance U.K.+E.U.

The mole is the amount of substance of a system which contains as many elementary entities as there are atoms in 0·012 kilogram of carbon 12.


When the mole is used, the elementary entities must be specified and may be atoms, molecules, ions, electrons, other particles, or specified groups of such particles.

(Fourteenth CGPM (1971), resolution 3).

Unit of luminous intensity U.K.+E.U.

The candela is the luminous intensity, in a given direction, of a source that emits monochromatic radiation of frequency of 540 × 10 12 hertz and that has a radiant intensity in that direction of (1/683) watt per steradian.

(Sixteenth CGPM (1979), resolution 3).

1.1.1. [F2Special name and symbol of the SI derived unit of temperature for expressing Celsius temperature] U.K.+E.U.
Quantity Unit
Name Symbol
Celsius temperature degree Celsius °C

[F3Celsius temperature [X2t] is defined as the difference [X2t = T - T 0] between the two thermodynamic temperatures [X2T] and [X2T 0 where T 0]  = 273,15 K. An interval or difference of temperature may be expressed either in kelvins or in degrees Celsius. The unit degree Celsius is equal to the unit kelvin .]

[F21.2. SI derived units] U.K.+E.U.
F41.2.1.SI supplementary unitsU.K.+E.U.

[ F4. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

F4. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .]

Definitions of SI supplementary units:U.K.+E.U.
Unit of plane angleU.K.+E.U.

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Unit of solid angleU.K.+E.U.

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

[F21.2.2. General rule for SI derived units U.K.+E.U.

Units derived coherently from SI base units are given as algebraic expressions in the form of products of powers of the SI base units with a numerical factor equal to 1.

1.2.3. SI derived units with special names and symbols U.K.+E.U.

Special names for the unit of power: the name volt–ampere (symbol VA ) when it is used to express the apparent power of alternating electric current, and var (symbol var ) when it is used to express reactive electric power. The var is not included in GCPM resolutions.

Quantity Unit Expression
Name Symbol In terms of other SI units In terms of SI base units
Plane angle radian rad m · m –1
Solid angle steradian sr m 2 · m –2
Frequency hertz Hz s –1
Force newton N m · kg · s –2
Pressure, stress pascal Pa N · m –2 m –1 · kg · s –2
Energy, work; quantity of heat joule J N · m m 2 · kg · s –2
Power a , radiant flux watt W J · s –1 m 2 · kg · s –3
Quantity of electricity, electric charge coulomb C s · A
Electric potential, potential difference, electromotive force volt V W · A –1 m 2 · kg · s –3 · A –1
Electric resistance ohm Ω V · A –1 m 2 · kg · s –3 · A –2
Conductance siemens S A · V –1 m –2 · kg –1 · s 3 · A 2
Capacitance farad F C · V –1 m –2 · kg –1 · s 4 · A 2
Magnetic flux weber Wb V · s m 2 · kg · s –2 · A –1
Magnetic flux density tesla T Wb · m –2 kg · s –2 · A –1
Inductance henry H Wb · A –1 m 2 · kg · s –2 · A –2
Luminous flux lumen lm cd · sr cd
Illuminance lux lx lm · m –2 m –2 · cd
Activity (of a radionuclide) becquerel Bq s –1
Absorbed dose, specific energy imparted, kerma, absorbed dose index gray Gy J · kg –1 m 2 · s –2
Dose equivalent sievert Sv J · kg –1 m 2 · s –2
Catalytic activity katal kat mol · s –1

Units derived from SI base units may be expressed in terms of the units listed in Chapter I.

In particular, derived SI units may be expressed by the special names and symbols given in the above table; for example, the SI unit of dynamic viscosity may be expressed as m –1 · kg · s –1 or N · s · m –2 or Pa · s.]

1.3. Prefixes and their symbols used to designate certain decimal multiples and submultiples U.K.+E.U.
[F3Factor Prefix Symbol
10 24 yotta Y
10 21 zetta Z
10 18 exa E
10 15 peta P
10 12 tera T
10 9 giga G
10 6 mega M
10 3 kilo [X3k]
10 2 hecto [X3h]
10 1 deca da
10 -1 deci d
10 -2 centi c
10 -3 milli m
10 -6 micro μ
10 -9 nano n
10 -12 pico p
10 -15 femto f
10 -18 atto a
10 -21 zepto z
10 -24 yocto y]

The names and symbols of the decimal multiples and submultiples of the unit of mass are formed by attaching prefixes to the word gram and their symbols to the symbol g .

Where a derived unit is expressed as a fraction, its decimal multiples and submultiples may be designated by attaching a prefix to units in the numerator or the denominator, or in both these parts.

Compound prefixes, that is to say prefixes formed by the juxtaposition of several of the above prefixes, may not be used.

1.4. Special authorized names and symbols of decimal multiples and submultiples of SI units U.K.+E.U.

The two symbols I and L may be used for the litre unit.

(Sixteenth CGPM (1979), resolution 6).


Unit listed in the International Bureau of Weights and Measures booklet as among the units to be permitted temporarily.

Quantity Unit
Name Symbol Value
Volume litre 1 or L a 1 l = 1 dm 3 = 10 -3 m 3
Mass tonne t 1 t = 1 Mg = 10 3 kg
Pressure, stress bar bar b 1 bar = 10 5 Pa
Note: U.K.+E.U.

The prefixes and their symbols listed in 1.3 may be used in conjunction with the units and symbols contained in Table 1.4.] U.K.+E.U.

Back to top


Print Options


Legislation is available in different versions:

Latest Available (revised):The latest available updated version of the legislation incorporating changes made by subsequent legislation and applied by our editorial team. Changes we have not yet applied to the text, can be found in the ‘Changes to Legislation’ area.

Original (As adopted by EU): The original version of the legislation as it stood when it was first adopted in the EU. No changes have been applied to the text.


See additional information alongside the content

Geographical Extent: Indicates the geographical area that this provision applies to. For further information see ‘Frequently Asked Questions’.

Show Timeline of Changes: See how this legislation has or could change over time. Turning this feature on will show extra navigation options to go to these specific points in time. Return to the latest available version by using the controls above in the What Version box.


Opening Options

Different options to open legislation in order to view more content on screen at once


More Resources

Access essential accompanying documents and information for this legislation item from this tab. Dependent on the legislation item being viewed this may include:

  • the original print PDF of the as adopted version that was used for the EU Official Journal
  • lists of changes made by and/or affecting this legislation item
  • all formats of all associated documents
  • correction slips
  • links to related legislation and further information resources

Timeline of Changes

This timeline shows the different versions taken from EUR-Lex as well as any subsequent versions created after exit day as a result of changes made by UK legislation.

The dates for the EU versions are taken from the document dates on EUR-LEx and may not always coincide with when the changes came into force for the document.

For any versions created after exit day as a result of changes made by UK legislation the date will coincide with the earliest date on which the change (e.g an insertion, a repeal or a substitution) that was applied came into force. For further information see our guide to revised legislation on Understanding Legislation.


More Resources

Use this menu to access essential accompanying documents and information for this legislation item. Dependent on the legislation item being viewed this may include:

  • the original print PDF of the as adopted version that was used for the print copy
  • correction slips

Click 'View More' or select 'More Resources' tab for additional information including:

  • lists of changes made by and/or affecting this legislation item
  • confers power and blanket amendment details
  • all formats of all associated documents
  • links to related legislation and further information resources