The couplings of W bosons to leptons and quarks are calculated by making an ansatz for the composite-W-boson Bethe-Salpeter amplitude. The ansatz is constructed to be consistent in the nonrelativistic limit with the W-boson wave function as generated from a potential model. The γW0 mixing parameter λ and the weak coupling g are derived. The only free parameter is the mass of the leptonic scalar preon ms which is found to be ∼2 TeV to correctly reproduce sin2θW. It is shown that sin2θW is only indirectly related to the wave function at the origin.