In radiative heat transfer, a view factor, ${\displaystyle F_{A\rightarrow B}}$, is the proportion of the radiation which leaves surface ${\displaystyle A}$ that strikes surface ${\displaystyle B}$. In a complex 'scene' there can be any number of different objects, which can be divided in turn into even more surfaces and surface segments.

View factors are also sometimes known as configuration factors, form factors, angle factors or shape factors.

Radiation leaving a surface within an enclosure is conserved. Because of this, the sum of all view factors from a given surface, ${\displaystyle S_{i}}$, within the enclosure is unity as defined by the summation rule

$${\displaystyle \sum _{j=1}^{n}{F_{S_{i}\rightarrow S_{j}}}=1}$$

where ${\displaystyle n}$ is the number of surfaces in the enclosure. Any enclosure with ${\displaystyle n}$ surfaces has a total ${\displaystyle n^{2}}$ view factors.

For example, consider a case where two blobs with surfaces A and B are floating around in a cavity with surface C. All of the radiation that leaves A must either hit B or C, or if A is concave, it could hit A. 100% of the radiation leaving A is divided up among A, B, and C.

Confusion often arises when considering the radiation that arrives at a target surface. In that case, it generally does not make sense to sum view factors as view factor from A and view factor from B (above) are essentially different units. C may see 10% of A's radiation and 50% of B's radiation and 20% of C's radiation, but without knowing how much each radiates, it does not even make sense to say that C receives 80% of the total radiation.

The reciprocity relation for view factors allows one to calculate ${\displaystyle F_{i\rightarrow j}}$ if one already knows ${\displaystyle F_{j\rightarrow i}}$ and is given as