  The availability of a number of new interferometric measurements of Main
Sequence and subgiant stars makes it possible to calibrate the surface
brightness relations of these stars using exclusively direct angular diameter
measurements. These empirical laws allow to predict the limb darkened angular
diameters theta_LD of dwarfs and subgiants using their dereddened Johnson
magnitudes, or their effective temperature. The smallest intrinsic dispersions
of sigma < 1% on theta_LD are obtained for the relations based on the K and L
magnitudes, for instance log theta_LD = 0.0502 (B-L) + 0.5133 - 0.2 L or log
theta_LD = 0.0755 (V-K) + 0.5170 - 0.2 K. Our calibrations are valid between
the spectral types A0 and M2 for dwarf stars (with a possible extension to
later types when using the effective temperature), and between A0 and K0 for
subgiants. Such relations are particularly useful to estimate the angular size
of calibrators for long baseline interferometry from readily available
broadband photometry.
