Maximum distances at which an ultra-cool dwarf can be detected by Gaia at the limiting magnitud G = 20 as a function of its absolute magnitude in the I band. These have been derived from BT-Settl models (filled circles) and the continuous lines represent the interpolation used in deriving the expected counts per spectral type bin in Table 1. The black continuous line corresponds to log   (g) = 5.0 and the blue line to log   (g) = 3.5. The top axis shows the effective temperature measured in Kelvin for a log   (g) = 5.0 object with the absolute I magnitudes shown in the x axis, according to the BT-Settl models. The T_eff – M_I mapping is only bi-valued below 600 K.