  This work explores the mixing rate of metals in the interstellar medium
(ISM), comparing observational constraints from our solar neighbourhood to high
resolution cosmological hydrodynamical simulations of Milky Way (MW)-like
galaxies. The mixing rate, described by the coefficient C, is varied in
simulations between 0 and 0.05, with resultant simulated galaxies compared to
observations of metallicity dispersion in young star clusters, HII regions and
neutral gas in the disc of the MW. A value of C between 0.003125 and 0.0125 is
found to self-consistently match a range of observables, with a best estimate
of C=0.0064$\pm$0.0004. We demonstrate that the relationship between metal
dispersion in young stars, HII regions and neutral gas, versus the coefficient
C, can be described by a power law. These constrained mixing rates infer a
comparatively well mixed ISM in the solar neighbourhood, at odds with some
recent observations that have reported a highly inhomogeneous ISM. The degree
of mixing suggested by this work is lower than what often employed in many
hydrodynamical simulations. Our results have implications for studying the
metallicity distribution of stars as well as of gas in the interstellar and
circumgalactic media.
