  The Hall scattering factor, $r$, is a key quantity for establishing carrier
concentration and drift mobility from Hall measurements; in experiments it is
usually assumed to be 1. In this paper we use a combination of analytical and
\textit{ab initio} modelling to determine $r$ in graphene. While at high
carrier densities $r \approx 1$ in a wide temperature range, at low doping the
temperature dependence of $r$ is very strong with values as high as 4 below
300~K. These high values are due to the linear bands around the Dirac cone and
the carrier scattering rates due to acoustic phonons. At higher temperatures
$r$ can instead become as low as $0.5$ due to the contribution of both holes
and electrons and the role of optical phonons. Finally, we provide a simple
analytical model to compute accurately $r$ in graphene in a wide range of
temperatures and carrier densities.
