Ratio F{sub K}/F{sub {pi}}in QCD

We determine the ratio F{sub K}/F{sub {pi}}in QCD with N{sub f}=2+1 flavors of sea quarks, based on a series of lattice calculations with three different lattice spacings, large volumes, and a simulated pion mass reaching down to about 190 MeV. We obtain F{sub K}/F{sub {pi}=}1.192(7){sub stat}(6){sub syst}. This result is then used to give an updated value of the Cabibbo-Kobayashi-Maskawa matrix element |V{sub us}|. The unitarity relation for the first row of this matrix is found to be well observed.