# 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 It was superseded in 2006 by ISO 80000-3 and again in 2019 by ISO 80000-3:2019.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 | Quantity | Unit | Unit | Unit | Remarks |
| Name | Symbol | Name | Symbol | Definition | Remarks |
| angle, (plane angle) | α, β, γ, θ, φ | radian | rad | 1 rad = 1 m⁄m = 1 | |
| angle, (plane angle) | α, β, γ, θ, φ | 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′) |
| angle, (plane angle) | α, β, γ, θ, φ | minute | ′ | 1′ = (1⁄60)° | 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′) |
| angle, (plane angle) | α, β, γ, θ, φ | second | ″ | 1″ = (1⁄60)′ | 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′) |
| solid angle | Ω | steradian | sr | 1 sr = 1 m$^2$⁄m$^2$ = 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 | 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 | |
| height | h | 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 | |
| thickness | d, δ | 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 | |
| radius | r, R | 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 | |
| diameter | d, D | 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 | |
| length of path, (linear) displacement vector | s, s | 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 | |
| distance | d, r | 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 | |
| cartesian coordinates | x, y, z | 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 | |
| radius of curvature | ϱ | 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 | |
| curvature | ϰ | reciprocal metre | m$^{−1}$ | | |
| area | A | square metre | m$^2$ | | The units are (1 a = 100 m$^2$) and hectare (1 ha = 100 a) are used for agrarian areas. |
| volume | V | cubic metre | m$^3$ | | |
| volume | V | litre | L, l | 1 L = 1 dm$^3$ | 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. |
| time, time interval, duration | t | minute | min | 1 min = 60 s | Representations of time of day are defined in ISO 8601. |
| time, time interval, duration | t | hour | h | 1 h = 60 min = 3 600 s | Representations of time of day are defined in ISO 8601. |
| time, time interval, duration | t | day | d | 1 d = 24 h = 1 440 min = 86 400 s | Representations of time of day are defined in ISO 8601. |
| angular velocity | ω | radian per second | rad⁄s | | ω=dφ⁄dt |
| angular acceleration | α | radian per second squared | rad⁄s$^2$ | | 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 | | |
| velocity | v (general symbol), v (velocity vector), c (propagation speed of waves), u, v, w (components of a velocity c) | kilometre per hour | km⁄h | | |
| acceleration, acceleration vector | a, a | metre per second squared | m⁄s$^2$ | 1 g = 9.80665 m⁄s$^2$ | |
| acceleration of free fall, acceleration due to gravity | g | metre per second squared | m⁄s$^2$ | 1 g = 9.80665 m⁄s$^2$ | |

## 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.