  We determine the weak mixing angle in the MS-bar scheme at energy scales
relevant for present and future low energy electroweak measurements. We relate
the renormalization group evolution of the weak mixing angle to the
corresponding evolution of the QED coupling and include higher-order terms in
alpha_s and alpha that had not been treated in previous analyses. We also
up-date the analysis of non-perturbative hadronic contributions and argue that
the associated uncertainty is small compared to anticipated experimental
errors. The resulting value of the low-energy weak mixing angle is sin^2
theta_W (0) = 0.23867 +- 0.00016.
