Atmospheric parameters determined via spectral modelling are unavailable for many of the known magnetic early B-type stars. We utilized high-resolution spectra together with NLTE models to measure effective temperatures $T_{rm eff}$ and surface gravities $log{g}$ of stars for which these measurements are not yet available. We find good agreement between our $T_{rm eff}$ measurements and previous results obtained both photometrically and spectroscopically. For $log{g}$, our results are compatible with previous spectroscopic measurements; however, surface gravities of stars previously determined photometrically have been substantially revised. We furthermore find that $log{g}$ measurements obtained with HARPSpol are typically about 0.1 dex lower than those from comparable instruments. Luminosities were determined using Gaia Data Release 2 parallaxes. We find Gaia parallaxes to be unreliable for bright stars ($V<6$ mag) and for binaries; in these cases we reverted to Hipparcos parallaxes. In general we find luminosities systematically lower than those previously reported. Comparison of $log{g}$ and $log{L}$ to available rotational and magnetic measurements shows no correlation between either parameter with magnetic data, but a clear slow-down in rotation with both decreasing $log{g}$ and increasing $log{L}$, a result compatible with the expectation that magnetic braking should lead to rapid magnetic spindown that accelerates with increasing mass-loss.
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