  A measurement of the production cross-section of Z bosons in pp collisions at
$\sqrt{s} = 13$ TeV is presented using dimuon and dielectron final states in
LHCb data. The cross-section is measured for leptons with pseudorapidities in
the range $2.0 < \eta < 4.5$, transverse momenta $p_\text{T} > 20$ GeV and
dilepton invariant mass in the range $60<m(\ell\ell)<120$ GeV. The integrated
cross-section from averaging the two final states is
\begin{equation*}\sigma_{\text{Z}}^{\ell\ell} = 194.3 \pm 0.9 \pm 3.3 \pm
7.6\text{ pb,}\end{equation*} where the first uncertainty is statistical, the
second is due to systematic effects, and the third is due to the luminosity
determination. In addition, differential cross-sections are measured as
functions of the Z boson rapidity, transverse momentum and the angular variable
$\phi^*_\eta$.
