  For accurate determination of particle masses accurate knowledge of the
momentum scale of the detectors is crucial. The procedure used to calibrate the
momentum scale of the LHCb spectrometer is described and illustrated using the
performance obtained with an integrated luminosity of $1.6~ fb^{-1}$ collected
during 2016 in $pp$ running. The procedure uses large samples of $J/\psi
\rightarrow \mu^+ \mu^-$ and $B^+ \rightarrow J/\psi K^+$ decays and leads to a
relative accuracy of $3 \times 10^{-4}$ on the momentum scale.
