  A good correlation has been found in star-forming galaxies between the soft
X-ray and the far infrared or radio luminosities. In this work we analyze the
relation between the soft X-ray and far infrared luminosities as predicted by
evolutionary population synthesis models, aiming first to test the validity of
the soft X-ray luminosity as a star formation rate estimator, using the already
known calibration of the FIR luminosity as a proxy, and second proposing a
calibration based on the predictions of evolutionary synthesis models. We have
computed the soft X-ray and far infrared luminosities expected for a massive
starburst as a function of evolutionary state, efficiency of the conversion of
mechanical energy into soft X-ray luminosity, star formation regime
(instantaneous or extended) and dust abundance, and have compared the
predictions with observational values for a sample of 62 star-forming galaxies
taken from the literature. The observational Lsoftx/Lfir ratios are consistent
with the predictions by the models under realistic assumptions (young
starbursts, efficiency in the re-processing of mechanical energy of few
percent), confirming the direct relation of the diffuse soft X-ray emission
with the star formation episode. The soft X-ray emission of the diffuse,
extended gas surrounding massive star forming regions can be used as a Star
Formation Rate estimator. The empirical calibrations presented in the
literature are supported by the predictions of evolutionary synthesis models
and by the analysis of a larger sample of star-forming galaxies, but it is
biased towards galaxies dominated by relatively unevolved starbursts.
