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′) | ||
| arcminute | ′ | 1′ = (1/60)° | |||
| arcsecond | ″ | 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 | 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 | The capital L is prefered 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 | 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 | ||
| 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 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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