  The SKA and its pathfinders will enable studies of HI emission at higher
redshifts than ever before. In moving beyond the local Universe, this will
require the use of cosmologically appropriate formulae that have traditionally
been simplified to their low-redshift approximations. In this paper, we
summarise some of the most important relations for tracing HI emission in the
SKA era, and present an online calculator to assist in the planning and
analysis of observations (hifi.icrar.org).
