Root mean square magnetic field from beam power
= Root mean square magnetic field from beam power
{title2=$B_{\rm rms}=\sqrt{\mu_0P/(cA)}$}
For a uniform plane-wave beam of cross section $A$, mean <intensity> is $P/A=cB_{\rm rms}^2/\mu_0$. Hence $B_{\rm rms}=\sqrt{\mu_0P/(cA)}$. A sinusoidal peak <amplitude> is $\sqrt2$ times the rms <amplitude>. A <Gaussian beam> requires a stated waist or diameter convention before inferring its on-axis field from total <power>.