The most precise measurement to date of the W boson hadronic decay branching fraction ratio 𝑅 c = ℬ (W →
cq)∕ ℬ (W → qq ) is presented. The measurement is based on a sample of proton-proton collision data from the
CERN LHC collected by the CMS experiment at a center-of-mass energy of 13 TeV in 2016--2018 with an integrated
luminosity of 138 fb −1 . The large cross section of top quark-antiquark production at the LHC offers a sizable high­
purity sample of W bosons suitable for this measurement. Events with one charged lepton (electron or muon)
and at least four jets, two tagged as bottom quark jets, are analyzed. Charm jets are tagged using the presence
of a muon inside the jet. The result, 𝑅 c = 0.489 ± 0.020 , is consistent with the standard model prediction and is
twice as precise as the current world-average value.