Box diagram in the elastic electron-proton scattering

We present an evaluation of the box diagram for elastic $\mathit{ep}$ scattering with the proton in the intermediate state. Using analytic properties of the proton form factors we express the amplitude via a twofold integral that involves the form factors in the spacelike region only. Therefore experimentally measured form factors can be used in the calculations directly. The numerical calculation is done with the form factors extracted by Rosenbluth separation, as well as polarization transfer method. The dependence of the results on the form factor choice is small for $Q^2\lesssim 6{\mathrm{GeV}}^2$ but becomes sizable at higher $Q^2$.