  We calculate the Casimir force and free energy for plane metallic mirrors at
non-zero temperature. Numerical evaluations are given with temperature and
conductivity effects treated simultaneously. The results are compared with the
approximation where both effects are treated independently and the corrections
simply multiplied. The deviation between the exact and approximated results
takes the form of a temperature dependent function for which an analytical
expression is given. The knowledge of this function allows simple and accurate
estimations at the % level.
