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. It was superseded in 2006 by ISO 80000-3.


Its definitions include:

Quantity Unit Remarks
Name Symbol Name Symbol Definition
(plane angle)
α, β, γ, ϑ, φ radian rad 1 rad = 1 mm = 1
degree ° 1° = π180 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′ = (160
second 1″ = (160)′
solid angle Ω steradian sr 1 sr = 1  = 1
length l, L metre m The metre is the length of the path travelled by light in vacuum during a time interval of 1299 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 ϰ=1ϱ
area A square metre The units are (1 a = 100 m²) and hectare (1 ha = 100 a) are used for agrarian areas.
volume V cubic metre
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 = 86 400 s
angular velocity ω radian per second rads ω=dφdt
angular acceleration α radian per second squared rad α=dωdt
velocity v (general symbol),
v (velocity vector),
c (propagation speed of waves),
u, v, w (components of a velocity c)
metre per second ms
kilometre per hour kmh
acceleration, acceleration vector a, a metre per second squared m g = 9.80665 m
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.