ISO 31-1

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ISO 31-1 is the part of international standard ISO 31 that defines names and symbols for quantities and units related to space and time.

Its definitions include:

Quantity Unit Remarks
Name Symbol Name Symbol Definition
angle,
(plane angle) α, β, γ, θ, φ radian rad 1 rad = 1 m/m = 1
degree ° 1° = π/180 rad There is no space between the number and these superscript-style unit symbols. Decimal subdivision of degrees is preferable (i.e., 12.5° instead of 12°30')
minute ′ 1′ = (1/60)°
second ″ 1″ = (1/60)′
solid angle Ω steradian sr 1 sr = 1 m²/m² = 1
length l, L metre m The metre is the length of the path travelled by light in vacuum during a time interval of 1/299 792 458 of a second; see speed of light
breadth b
height h
thickness d, δ
radius r, R
diameter d, D
length of path s
distance d, r
cartesian coordinates x, y, z
radius of curvature ϱ
curvature ϰ reciprocal metre m⁻¹ ϰ=1/ϱ
area A square metre m² The units are (1 a = 100 m²) and hectare (1 ha = 100 a) are used for agrarian areas.
volume V cubic metre m³
litre l, L 1 l = 1 dm³
time, time interval, duration t second s 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 Representations of time of day are defined in ISO 8601.
minute min 1 min = 60 s
hour h 1 h = 60 min = 3 600 s
day d 1 d = 24 h = 86 400 s
angular velocity ω radian per second rad/s ω=dφ/dt
angular acceleration α radian per second squared rad/s² α=dω/dt
velocity v (general symbol),
c (propagation speed of waves),
u, v, w (vector components of a velocity c) metre per second m/s
acceleration a metre per second squared m/s²
acceleration of free fall, acceleration due to gravity g

Quantity	Unit	Remarks
Name	Symbol	Name	Symbol	Definition
angle, (plane angle)	α, β, γ, θ, φ	radian	rad	1 rad = 1 m/m = 1
degree	°	1° = π/180 rad	There is no space between the number and these superscript-style unit symbols. Decimal subdivision of degrees is preferable (i.e., 12.5° instead of 12°30')
minute	′	1′ = (1/60)°
second	″	1″ = (1/60)′
solid angle	Ω	steradian	sr	1 sr = 1 m²/m² = 1
length	l, L	metre	m	The metre is the length of the path travelled by light in vacuum during a time interval of 1/299 792 458 of a second; see speed of light
breadth	b
height	h
thickness	d, δ
radius	r, R
diameter	d, D
length of path	s
distance	d, r
cartesian coordinates	x, y, z
radius of curvature	ϱ
curvature	ϰ	reciprocal metre	m⁻¹		ϰ=1/ϱ
area	A	square metre	m²		The units are (1 a = 100 m²) and hectare (1 ha = 100 a) are used for agrarian areas.
volume	V	cubic metre	m³
litre	l, L	1 l = 1 dm³
time, time interval, duration	t	second	s	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	Representations of time of day are defined in ISO 8601.
minute	min	1 min = 60 s
hour	h	1 h = 60 min = 3 600 s
day	d	1 d = 24 h = 86 400 s
angular velocity	ω	radian per second	rad/s		ω=dφ/dt
angular acceleration	α	radian per second squared	rad/s²		α=dω/dt
velocity	v (general symbol), c (propagation speed of waves), u, v, w (vector components of a velocity c)	metre per second	m/s
acceleration	a	metre per second squared	m/s²
acceleration of free fall, acceleration due to gravity	g

Annex A of ISO 31-1 lists units of space and time based on the foot, pound and second. Annex B lists some other non-SI units of space and time, namely the gon, light year, astronomical unit, parsec, tropical year, and gal.

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