  When averaged over large scales, star formation in galaxies is observed to
follow the empirical Kennicutt-Schmidt (KS) law for surface densities above a
constant threshold. While the observed law involves surface densities,
theoretical models and simulations generally work with volume density laws
(i.e. Schmidt laws). We derive analytic relations between star formation laws
expressed in terms of surface densities, volume densities, and pressures and we
show how these relations depend on parameters such as the effective equation of
state of the multiphase interstellar medium. Our analytic relations enable us
to implement observed surface density laws into simulations. Because the
parameters of our prescription for star formation are observables, we are not
free to tune them to match the observations. We test our theoretical framework
using high-resolution simulations of isolated disc galaxies that assume an
effective equation of state for the multiphase interstellar medium. We are able
to reproduce the star formation threshold and both the slope and the
normalisation of arbitrary input KS laws without tuning any parameters and with
very little scatter, even for unstable galaxies and even if we use poor
numerical resolution. Moreover, we can do so for arbitrary effective equations
of state. Our prescription therefore enables simulations of galaxies to bypass
our current inability to simulate the formation of stars. On the other hand,
the fact that we can reproduce arbitrary input thresholds and KS laws, rather
than just the particular ones picked out by nature, indicates that simulations
that lack the physics and/or resolution to simulate the multiphase interstellar
medium can only provide limited insight into the origin of the observed star
formation laws.
