If thermal strain epsilon(th) exists in a thin film, then the lattice constant a is dependent on the orientation of the crystallites within the film due to the elastic anisotropy of the crystallites. For this case, in X-ray diffraction the lattice constant a is a function of the Miller indices (hkl) and of the measurement direction. Assuming the film to have cubic structure, a theoretical expression for the relative strain epsilon(hkl)/epsilon(th) is derived. The expression is valid for the usual Bragg-Brentano geometry and for grazing incidence diffraction. As an application, from the measured a(hkl) of a thin copper film, the film’s strain-free lattice constant a(0) and epsilon(th) are estimated.