A formula for quick calculation of thermal characteristics of materials used for thermal protection is proposed. The mode of heating of the sample external surface (subjected to thermal exposure) is approximated by two regions, which differ by corresponding boundary conditions on the heating surface: T_f=cτ (linear growing with time) and T_f=const (with permanent temperature of destruction). That approximation permits to obtain an analytical solution in integral form. In order to simplify and accelerate data treatment, a simple empirical formula is proposed. A contribution of each thermal region is proportional to the regime duration. A good agreement with an analytical solution is shown.