M_HI – redshift parameter space, showing where MIGHTEE-HI is sensitive to galaxies, divided into cells of size Δz = 0.01 and Δ log(M_HI) = 0.1. Light, medium, and dark grey shading indicates where at least one, 10, and 25 galaxies are expected per cell. The black, red, and blue curves correspond to the M_HI of galaxies as a function of redshift, for three fixed diameters, using the M_HI – D_HI relation from Wang et al. (2016). The black line shows the 12 arcsec MeerKAT beam, indicating galaxies below this mass will be unresolved. The red dashed line is for marginally resolved sources and the blue dashed line is for well-resolved sources with five beams across. 0.09 <  z <  0.22 is worst-afftected by RFI, and the size and shape of the synthesised beam is strongly affected in this region.