HI line flux (flux corresponding to the area under the 21-cm line profile) is generally expressed in converted in magnitude m21 according to the formula adopted in RC3: m21 = -2.5 log f + 17.40 where f is the area of the 21-cm line profile expressed in Jy*km/s (the constant is 16.6 if the flux is in 10**-22 W/m**2). Non-detections (or upper limits) are not stored in the database, therefore galaxies with no HI measurements available may either be too faint or not have been observed. ... HyperLeda . ...