A measurement is presented of the $Z/{\gamma }^{\ast }\to \tau \tau $ cross section in $\text{pp}$ collisions at $\sqrt{s}=13\phantom{\rule{0.333333em}{0ex}}\text{TeV}$ , using data recorded by the CMS experiment at the LHC, corresponding to an integrated luminosity of $2.3\phantom{\rule{0.333333em}{0ex}}{\text{fb}}^{-1}$ . The product of the inclusive cross section and branching fraction is measured to be $\sigma \left(\text{pp}\to Z/{\gamma }^{\ast }\text{+X}\right)\phantom{\rule{0.166667em}{0ex}}B\left(Z/{\gamma }^{\ast }\to \tau \tau \right)=1848±12\phantom{\rule{0.166667em}{0ex}}\left(\text{stat}\right)±67\phantom{\rule{0.333333em}{0ex}}\text{(syst \,+\,lumi) pb}$ , in agreement with the standard model expectation, computed at next-to-next-to-leading order accuracy in perturbative quantum chromodynamics. The measurement is used to validate new analysis techniques relevant for future measurements of $\tau $ lepton production. The measurement also provides the reconstruction efficiency and energy scale for $\tau $ decays to hadrons $\phantom{\rule{0.166667em}{0ex}}+\phantom{\rule{0.166667em}{0ex}}{\nu }_{\tau }$ final states, determined with respective relative uncertainties of 2.2 and 0.9%.