	Unit & Tolerance

International System of Units (SI)

Scope

(excerpt from JIS Z 8203-1985)

This standard specifies International System of Units (SI), the usage of units in accordance with SI, and the units used or usable with the units specified in SI.

Terminology and definition

The major terms and their definition used in this standard are as follows:

International System of Units(SI)

A coherent unit system adopted and recommended by the International Committee of Weights and Measures (CIPM).
The system consists of basic units, supplementary units, derived units which are the combination of basic and supplementary units, and prefixes indicating the power of 10. SI is an abbreviation for International System of Units.

SI units

A generic name for the basic units, supplementary units and derived units specified in International System of Units (SI).

SI units

Basic units

Table 1 lists the basic units.

Table 1 Basic units

Quantity	Unit	Symbol	Definition
Length	meter	m	A meter is the length of the path traveled by light in a vacuum during a time interval of 1/299,792,458 of a second.
Mass	kilogram	kg	A kilogram is the unit of mass (it is not the unit of weght or force); it is equal to the mass of the international prototype of the kilogram.
Time	second	s	A 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 cesium 133 atom.
Electric current	ampere	A	An ampere is that constant current which, if maintained in two straight parallel conductors of infinite length, of negligible circular cross-section, and placed 1 meter apart in vacuum, would produce between these conductors a force equal to 2 x 10^-7 newton per meter of length.
Thermodynamic temperature	kelvin	K	A kelvin is the fraction 1/273.16 of the thermodynamic temperature of the triple point of water.
Amount of substance	mole	mol	A mole is the amount of substance of a system which contains the same number of elementary entities or the group of elementary entities (the composition must be specified) as the number of atoms in 0.012 kilogram of carbon 12. When the mole is used, the elementary entities or the group of elementary entities must be specified.
Luminous intensity	candela	cd	A candela is the luminous intensity, in a given direction, of a source that emits monochromatic radiation of frequency 540 x 10¹² hertz and that has a radiant intensity in that direction of 1/683 watt per steradian.

Supplementary units

Table 2 lists the supplementary units.

Table 2 Supplementary units

Quantity	Unit	Symbol	Definition
Plane angle	radian	rad	A radian is the plane angle formed at the center of a circle by two radii which are determined by an arc of the circle equal in length to the radius.
Solid angle	steradian	sr	A steradian is the solid angle whose vertex is the center of the sphere and which is determined by the area on the sphere surface that has the equal area to the square whose one side is equal in length to the radius.

Derived units

Derived units are the units which may be expressed in terms of base units and supplementary units by means of algebra (using the mathematical symbols of multiplication and division). Table 3 lists the derived units which have special names.

Table 3 Examples of derived units expressed in terms of base units

Derived quantity	Name	Symbol
area	square meter	m²
volume	cubic meter	m³
speed, velocity	meter per second	m/s
acceleration	meter per second squared	m / s²
wave number	reciprocal meter	m^-1
density	kilogram per cubic meter	kg/m³
current density	ampere per square meter	A/m²
magnetic field strength	ampere per meter	A/m
concentration (of amount of substance)	mole per cubic meter	mol/m³
specific volume	cubic meter per kilogram	m³/kg
luminance	candela per square meter	cd/m²

Table 3 Derived units with special names

Quantity	Unit name	Unit symbol	Basic unit or assembly method by supplementary unit or other assembly unit
Frequency	Hertz	Hz	1Hz＝1s^-1
Force	Newton	N	1N＝1kg • m/s²
Pressure, stress	Pascal	Pa	1Pa＝1N/m²
Energy, work, calorie	Joule	J	1J＝1N • m
Power, rate of production motive energy, electric power	watt	W	1W＝1J/s
Electric charge, quantity of electricity	Coulomb	C	1C＝1A • s
potential, potential difference, voltage, electromotive force	Voltage	V	1V＝1J/C
electric capacity, capacitance	Farad	F	1F＝1C/V
(Electric) resistance	Ohm	Ω	1Ω＝1V/A
(Electrical) conductance	Siemens	S	1S＝1Ω^-1
Flux	Weber	Wb	1Wb＝1V • s
Magnetic flux density, magnetic induction	Tesla	T	1T＝1Wb/m²
Inductance	Henry	H	1H＝1Wb/A
Celsius temperature	Celsius degree or degree	ºC	1t＝T-To
Luminous flux	Lumen	lm	1lm＝1cd • sr
Illumination	Lux	lx	1lx＝1lm/m²

Integer multiplication of SI unit 10

Prefix

Multiple number which constitutes integer multiplication of SI unit 10, prefix name, and prefix symbol are as specified in Table 4.

Table 4. Prefix

Multiple number	Prefix	Symbol	Multiple number	Prefix	Symbol
10¹⁸	Exa	E	10^-1	Deci	d
10¹⁵	Peta	P	10^-2	Centi	c
10¹²	Tera	T	10^-3	Milli	m
10⁹	Giga	G	10^-6	Micro	µ
10⁶	Mega	M	10^-9	Nano	n
10³	Kilo	k	10^-12	Pico	p
10²	Hecto	h	10^-15	Femto	f
10	Deca	da	10^-18	Ato	a

Units not Included in the SI

Important Units to be Used together with the SI Unit

Considering their practical importance, the units listed in Table 5 are used in addition to the SI unit although they are not included in the SII.

Table 5 Units to be Used together with the SI Unit

Quantity	Unit name	Unit symbol	Definition:
Time	Minute Hour Day	min h d	1min = 60s 1h = 60min 1d＝24h
Plane angle	Degree Arcmin Second	º ' "	1º = (Π/180)rad 1' = (1/60)º 1" = (1/60)'
Volume	Liter	l, L⁽³⁾	1l = 7dm³
Mass	Ton	t	1t = 10³kg


Note⁽³⁾	"L" can be used if it is possible that "l" is mistaken with something else.

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