  An exact formula for the temperature dependent Hall number of metals is
derived. It is valid for non-relativistic fermions or bosons, with arbitrary
potential and interaction. This DC transport coefficient is proven to
(remarkably) depend solely on equilibrium susceptibilities, which are more
amenable to numerical algorithms than the conductivity. An application to
strongly correlated phases is demonstrated by calculating the Hall sign in the
vicinity of Mott phases of lattice bosons.
