  Scale setting is of central importance in lattice QCD. It is required to
predict dimensional quantities in physical units. Moreover, it determines the
relative lattice spacings of computations performed at different values of the
bare coupling, and this is needed for extrapolating results into the continuum.
Thus, we calculate a new quantity, $w_0$, for setting the scale in lattice QCD,
which is based on the Wilson flow like the scale $t_0$ (M. Luscher, JHEP 1008
(2010) 071). It is cheap and straightforward to implement and compute. In
particular, it does not involve the delicate fitting of correlation functions
at asymptotic times. It typically can be determined on the few per-mil level.
We compute its continuum extrapolated value in 2+1-flavor QCD for physical and
non-physical pion and kaon masses, to allow for mass-independent scale setting
even away from the physical mass point. We demonstrate its robustness by
computing it with two very different actions (one of them with staggered, the
other with Wilson fermions) and by showing that the results agree for physical
quark masses in the continuum limit.
