Also known as the Divergence Theorem or Gauss's Theorem Converts a volume integral into a surface integral The volume integral of the divergence of a field variable is equal to the surface integral of the face normal component of that field variable: $ \int_V\nabla\cdot\boldsymbol{F}\,dV=\int_S\boldsymbol{F}\cdot\hat{\boldsymbol{n}}\,dS $ The accumulation of a substance inside a volume is equal to the flux of the substance through its boundaries/faces.