  Context. Precise and accurate determinations of effective temperature and
surface gravity are mandatory to derive reliable chemical abundances and
fundamental parameters like distances, masses, radii, luminosities of OB stars.
Aims. Atmospheric parameters recently determined at high precision with several
independent spectroscopic indicators in NLTE are employed to calibrate
photometric relationships. Methods. Temperatures and gravities of 30
calibrators are compared to reddening-independent quantities of the Johnson and
Stroemgren photometric systems. We also examine the spectral and luminosity
classification of the star sample and compute bolometric corrections. Results.
Calibrations of temperatures and gravities are proposed for various photometric
indices and spectral types. Effective temperatures can be determined at a
precision of ~400 K for luminosity classes III/IV and ~800 K for luminosity
class V. Surface gravities can reach internal uncertainties as low as ~0.08 dex
when using our calibration to the Johnson Q-parameter. Similar precision is
achieved for gravities derived from the beta-index and the precision is lower
for both atmospheric parameters when using the Stroemgren indices c1 and [u-b].
Our uncertainties are smaller than typical differences among other methods in
the literature, reaching values up to ~2000 K for temperature and ~0.25 dex for
gravity, and in extreme cases, ~6000 K and ~0.4 dex, respectively. A parameter
calibration for sub-spectral types is also proposed. We present a new
bolometric correction relation to temperature based on our empirical data.
Conclusions. The photometric calibrations presented here are useful tools to
estimate effective temperatures and surface gravities of non-supergiant OB
stars in a fast manner. We recommend to use these calibrations as a first step,
with subsequent refinements based on spectroscopy (abridged).
