# 8*pi in the Einstein field equations?

A typical formulation of the Einstein equations is

$$R_{\mu\nu}-\frac{1}{2}Rg_{\mu\nu}+\Lambda g_{\mu\nu}=\frac{8\pi G}{c^4}T_{\mu\nu}$$

The $\frac{G}{c^4}$ make the units work out. What about the 8*pi? Why is this necessary?