**ISO 80000-3:2006** is an "ISO standard entitled *Quantities and units – Part 3: Space and time*, superseding "ISO 31-1 and "ISO 31-2.^{[1]} It is a part of the group of standards called "ISO/IEC 80000, which together form the "International System of Quantities.

The standard is divided into the following chapters:

- Foreword
- Introduction

- Scope
- Normative references
- Names, symbols and definitions

- Annex A (informative) Units in the "CGS system with special names
- Annex B (informative) Units based on the "foot, "pound, "second, and some other related units
- Annex C (informative) Other non-SI units given for information, especially regarding the conversion factors

ISO 80000-3:2006 assigns names and symbols to quantities and units of space and time, and defines these quantities and units. For example,

- "metre (symbol m): "
*length of the path travelled by light in vacuum during a time interval of 1/(299 792 458) of a second*."

Other units of distance defined by the standard are the "ångström (symbol Å) (10^{−10} m) and the "nautical mile (1852 m). No symbol is given for the nautical mile.

The "are (symbol a) is a unit of area equal to 100 square metres.

The "litre (symbol L or l) is a unit of volume equal to 10^{−3} m^{3}.

- "radian
- "degree of arc: π/180 radians
- "minute of arc: 1/60 degrees of arc
- "second of arc: 1/60 minutes of arc
- "gon: π/200 "rad

- "curvature:
*χ*= 1/*ρ*, where*ρ*= "radius of curvature - "area
- "volume
- "angle:
*α*=*s*/*r*, where*s*= length of "arc of a circle and*r*= "radius of the circle - "solid angle

ISO 80000-3:2006 assigns names and symbols to quantities and units of space and time, and defines these quantities and units. For example,

- "second (symbol s): "
*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*"

Other units of time defined by the standard include the "minute (1 min = 60 s), the "hour (1 h = 60 min) and the "day (1 d = 24 h). The "year (1 a = 365 d or 366 d) is defined in an informative annex.

- "knot (symbol
**kn**): one nautical mile per hour = 1852/3600 m/s ≈ 0.514 444 m/s.

The "standard acceleration of free fall is 9.806 65 m/s^{2}.

Units of *"level* defined by ISO 80000-3:2006 are:

- "neper: 1 Np = ln(e) = 1
- "bel: 1 B = ln(√10) Np ≈ 1.1512925464970228420089957273422 Np
- "decibel: 1 dB = 0.1 B

The following CGS units are deprecated:

The following non-SI units are deprecated by the standard:

- The "inch ("Imperial unit of distance, equal to 25.4 mm)
- The "foot (Imperial unit of distance, equal to 12 inches)
- The "yard (Imperial unit of distance, equal to 3 "feet)
- The "mile (Imperial unit of distance, equal to 1760 yards)
- The "acre (Imperial unit of area, equal to 4840 square yards)

The following non-SI units are given for conversion. They are not deprecated by the standard:

- Units of distance: "light year (
*the distance travelled in one year by light in vacuum*; abbreviation**l.y.**; 1 l.y. ≈ 9.460 730 × 10^{15}m), "astronomical unit (*the mean distance of the Earth from the Sun*, 1 ua ≈ 1.495 978 706 91(30) × 10^{11}m), "parsec (*the distance at which 1 ua subtends an angle of 1 " (second of arc)*; 1 pc ≈ 206 264.8 ua ≈ 30.856 78 × 10^{15}m) - Units of time: "year (a), equal to either 365 days or 366 days

**^**"ISO 80000-3:2006". "International Organization for Standardization. Retrieved 20 July 2013.