ISO 31-1

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. It was superseded in 2006 by ISO 80000-3 and again in 2019 by ISO 80000-3:2019.

Definitions

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, (linear) displacement vector	s, s
distance	d, r
cartesian coordinates	x, y, z
radius of curvature	ϱ
curvature	ϰ	reciprocal metre	m−1
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	The capital L is preferred as the small l can be indistinguishable from the number 1 in some fonts.
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 = 1 440 min = 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), v (velocity vector), c (propagation speed of waves), u, v, w (components of a velocity c)	metre per second	m⁄s
		kilometre per hour	km⁄h
acceleration, acceleration vector	a, a	metre per second squared	m⁄s2	1 g = 9.80665 m⁄s2
acceleration of free fall, acceleration due to gravity	g

Annex A

Annex A of ISO 31-1 lists units of space and time based on the foot, pound, and second.

Annex B

Annex B lists some other non-SI units of space and time, namely the gon, light year, astronomical unit, parsec, tropical year, and gal.