The W boson helicity fractions from top quark decays in View the MathML sourcett‾ events are measured using data from proton–proton collisions at a centre-of-mass energy of View the MathML source8TeV. The data were collected in 2012 with the CMS detector at the LHC, corresponding to an integrated luminosity of View the MathML source19.8fb−1. Events are reconstructed with either one muon or one electron, along with four jets in the final state, with two of the jets being identified as originating from b quarks. The measured helicity fractions from both channels are combined, yielding View the MathML sourceF0=0.681±0.012(stat)±0.023(syst), View the MathML sourceFL=0.323±0.008(stat)±0.014(syst), and View the MathML sourceFR=−0.004±0.005(stat)±0.014(syst) for the longitudinal, left-, and right-handed components of the helicity, respectively. These measurements of the W boson helicity fractions are the most accurate to date and they agree with the predictions from the standard model.