We derive our own implementation of the InfraRed Flux Method (IRFM) to recover, from multi-band photometry, accurate effective temperatures, bolometric luminosities and angular diameters of such stars. We find very tight relations between photometric indices and the fundamental physical parameters of the stars. In particular, the relation between angular diameters and J-band index shows remarkably small scatter. Our IRFM temperature scale is 100 K hotter than recent analogous determinations in the literature, but is in agreement with spectroscopically calibrated temperature scales. Our smaller angular diameters are more in line with the limb-darkening correction predicted by the latest 3D model atmospheres and also with asteroseismology.