A precision measurement of the ZZ boson production cross-section at s=13s ​=13 TeV in the forward region is presented, using pppp collision data collected by the LHCb detector, corresponding to an integrated luminosity of 5.1 fb−1−1. The production cross-section is measured using Z→μ+μ−Z→μ+μ− events within the fiducial region defined as pseudorapidity 2.0<η<4.52.0<η<4.5 and transverse momentum pT>20pT​>20 GeV/cc for both muons and dimuon invariant mass 60