The Poynting vector is used to define power patterns in electromagnetic waves. It points in the direction of the wave propagation and its amplitude is calculated by multiplying the amplitudes of the electric and magnetic fields divided by the permeability of the medium in which the wave flows.
S = 1/u * EB
S is the Poynting vector amplitude
E = the electric field amplitude
B = the magnetic field amplitude
u = the permeability of the medium (4*PI* 10^-7 H/m for vacuum)