  This paper presents the electron and photon energy calibration obtained with
the ATLAS detector using about 36 fb$^{-1}$ of LHC proton-proton collision data
recorded at $\sqrt{s}=13$ TeV in 2015 and 2016. The different calibration steps
applied to the data and the optimization of the reconstruction of electron and
photon energies are discussed. The absolute energy scale is set using a large
sample of $Z$ boson decays into electron--positron pairs. The systematic
uncertainty in the energy scale calibration varies between 0.03% to 0.2% in
most of the detector acceptance for electrons with transverse momentum close to
45 GeV. For electrons with transverse momentum of 10 GeV the typical
uncertainty is 0.3% to 0.8% and it varies between 0.25% and 1% for photons with
transverse momentum around 60 GeV. Validations of the energy calibration with
$J/\psi \rightarrow e^+e^-$ decays and radiative $Z$ boson decays are also
presented.
