The W boson helicity fractions from top quark decays in (t\overline{t}) events are measured using data from proton–proton collisions at a centre-of-mass energy of 8TeV. The data were collected in 2012 with the CMS detector at the LHC, corresponding to an integrated luminosity of 19.8fb⁻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 (F_{0}) = 0.681 ± 0.012 (stat) ± 0.023 (syst), (F_{L}) = 0.323 ± 0.008 (stat) ± 0.014 (syst), and (F_{R}) = −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.