 Root mean square magnetic field from beam power

For a uniform plane-wave beam of cross section $A$, mean intensity is $P/A=c{B}_{\mathrm{rms}}^2/{\mu }_0$. Hence $B_{\mathrm{rms}}=\sqrt{{\mu }_{0}P/(cA)}$. A sinusoidal peak amplitude is $\sqrt{2}$ times the rms amplitude. A Gaussian beam requires a stated waist or diameter convention before inferring its on-axis field from total power.