  Star formation laws are empirical relations between the cold gas (HI+H$_2$)
content of a galaxy and its star formation rate (SFR), being crucial for any
model of galaxy formation and evolution. A well known example of such laws is
the Schmidt-Kennicutt law, which is based on the projected surface densities.
However, it has been long unclear whether a more fundamental relation exists
between the intrinsic volume densities. By assuming the vertical hydrostatic
equilibrium, we infer radial profiles for the thickness of gaseous discs in a
sample of 23 local galaxies, and use these measurements to convert the observed
surface densities of the gas and the SFR into the de-projected volume
densities. We find a tight correlation linking these quantities, that we call
the volumetric star formation law. This relation and its properties have
crucial implications for our understanding of the physics of star formation.
