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 It was superseded in 2006 by ISO 80000-3.

## Definitions

Its definitions include:

Quantity Unit Remarks
NameSymbolNameSymbolDefinition
angle,
(plane angle)
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′)
minute1′ = (160
second1″ = (160)′
solid angleΩsteradiansr1 sr = 1  = 1
lengthl, L metre m The metre is the length of the path travelled by light in vacuum during a time interval of 1299792458 of a second; see speed of light
heighth
thicknessd, δ
diameterd, D
length of path, (linear) displacement vectors, s
distanced, r
cartesian coordinatesx, y, z
curvatureϰreciprocal metrem−1 ϰ=1ϱ
areaAsquare metre The units are (1 a = 100 m²) and hectare (1 ha = 100 a) are used for agrarian areas.
volume V cubic metre
litreL, l1 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 secondsThe second is the duration of 9192631770 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.
minutemin1 min = 60 s
hourh1 h = 60 min = 3600 s
dayd1 d = 24 h = 86400 s
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 vectora, a metre per second squared m 1 g = 9.80665 m
acceleration of free fall, acceleration due to gravityg

## 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.