The steradian (symbol: sr) is the SI unit of solid angle. It is used to quantify two-dimensional angular spans in three-dimensional space, analogously to how the radian quantifies angles in a plane. The name is derived from the Greek stereos for "solid" and the Latin radius for "ray, beam".
The steradian, like the radian, is dimensionless, essentially because a solid angle is the ratio between the area subtended and the square of its distance from the vertex: both the numerator and denominator of this ratio have dimension length2. It is useful, however, to distinguish between dimensionless quantities of different nature, so in practice the symbol "sr" is used to indicate a solid angle. For example, radiant intensity can be measured in watts per steradian (W·sr−1). The steradian was formerly an SI supplementary unit, but this category was abolished from the SI in 1995 and the steradian is now considered an SI derived unit.
A steradian can be defined as the solid angle subtended at the center of a unit sphere by a unit area on its surface. For a general sphere of radius r, any portion of its surface with area A = r2 subtends one steradian.
Because the surface area of a sphere is 4πr2, the definition implies that a sphere measures 4π ≈ 12.56637 steradians. By the same argument, the maximum solid angle that can be subtended at any point is 4π sr. A steradian can also be called a squared radian.
Other properties 
Since A = r2, it corresponds to the area of a spherical cap (A = 2πrh) (wherein h stands for the "height" of the cap), and the relationship h/r = 1/(2π) holds. Therefore one steradian corresponds to the plane (i.e. radian) angle of the cross-section of a simple cone subtending the plane angle 2θ, with θ given by:
This angle corresponds to the plane aperture angle of 2θ ≈ 1.144 rad or 65.54°.
The solid angle in steradians of a cone whose cross-section subtends the angle 2θ (θ shown in the image) is:
This can be re-expressed as the πR^2 circular area of the base of the cone created by θ:
Analogue to radians 
In two dimensions, the angle in radians is related to the arc length it cuts out:
- l is arc length, and
- r is the radius of the circle.
Now in three dimensions, the solid angle in steradians is related to the area it cuts out:
- S is the surface area, and
- r is the radius of the sphere.
SI multiples 
Steradians only go up to 4π ≈ 12.56637, so the large multiples are not usable for the base unit, but could show up in such things as rate of coverage of solid angle, for example.
|Multiple||Name||Symbol||May be visualized as...|
|101||decasteradian||dasr||Surface area of the Americas plus liquid water on Earth, relative to Earth (cyan on map)|
|100||steradian||sr||Area of Oceania plus Asia excluding Russia, relative to Earth (yellow on map)|
|10−1||decisteradian||dsr||Area of Algeria plus Libya, relative to Earth (green on map)|
|10−2||centisteradian||csr||Area of Zimbabwe, relative to Earth (blue on map)|
|10−3||millisteradian||msr||Area of Switzerland, relative to Earth (red on map)|
|10−6||microsteradian||µsr||Area of Costa Mesa, California, relative to Earth|
|10−9||nanosteradian||nsr||About 8 American football fields, relative to Earth|
|10−12||picosteradian||psr||Area of a small apartment, relative to Earth|
|10−15||femtosteradian||fsr||Area of a sheet of A5 paper, relative to Earth|
|10−18||attosteradian||asr||Area of a quarter-inch square, relative to Earth|
|10−21||zeptosteradian||zsr||Cross-sectional area of 32 gauge wire, relative to Earth|
|10−24||yoctosteradian||ysr||Surface area of a red blood cell, relative to Earth|
- "Steradian", McGraw-Hill Dictionary of Scientific and Technical Terms, fifth edition, Sybil P. Parker, editor in chief. McGraw-Hill, 1997. ISBN 0-07-052433-5.
- 10.0 sr (404 million km² out of 510 million km²)
- 1.01 sr (40.8 million km² out of 510 million km²)
- 0.102 sr (4.14 million km² out of 510 million km²)
- 0.00963 sr (391 000 km² out of 510 million km²); Paraguay, at 0.0100 sr (407 000 km²) is closer to 1 csr, but has been shaded for the 10 sr region as part of the Americas
- 0.00102 sr (41 300 km² out of 510 million km²)
- 0.00000100 sr (40.7 km² out of 510 million km²)