SI unit of solid angle

The steradian (symbol: sr) or square radian is an SI unit. It is like a radian, but for smooth 3D objects, for example, a sphere. A steradian is about one twelfth the surface area of a sphere, regardless of how big the sphere is.[1]

• The surface area of a sphere is ${\displaystyle 4\pi r^{2}}$
• The surface area of a steradian is just ${\displaystyle r^{2}}$