We present a new luminosity-colour relation based on trigonometric parallaxes for thin-disc main-sequence stars in Sloan Digital Sky Survey (SDSS) photometry. We matched stars from the newly reduced Hipparcos catalogue with the ones taken from Two-Micron All-Sky Survey (2MASS) All-Sky Catalogue of Point Sources, and applied a series of constraints, i.e. relative parallax errors (σπ/π <= 0.05), metallicity (-0.30 <= [M/H] <= 0.20dex), age (0 <= t <= 10Gyr) and surface gravity (logg > 4), and obtained a sample of thin-disc main-sequence stars. Then, we used our previous transformation equations (Bilir et al. 2008a) between SDSS and 2MASS photometries and calibrated the Mg absolute magnitudes to the (g - r)0 and (r - i)0 colours. The transformation formulae between 2MASS and SDSS photometries along with the absolute magnitude calibration provide space densities for bright stars which saturate the SDSS magnitudes.