  A combination of fifteen top quark mass measurements performed by the ATLAS
and CMS experiments at the LHC is presented. The data sets used correspond to
an integrated luminosity of up to 5 and 20$^{-1}$ of proton-proton collisions
at center-of-mass energies of 7 and 8 TeV, respectively. The combination
includes measurements in top quark pair events that exploit both the
semileptonic and hadronic decays of the top quark, and a measurement using
events enriched in single top quark production via the electroweak $t$-channel.
The combination accounts for the correlations between measurements and achieves
an improvement in the total uncertainty of 31% relative to the most precise
input measurement. The result is $m_\mathrm{t}$ = 172.52 $\pm$ 0.14 (stat)
$\pm$ 0.30 (syst) GeV, with a total uncertainty of 0.33 GeV.
