A new approximate method for calculating the opacity of the atomic lines in the computation of stellar model atmospheres is presented. Transforming the sums in the equation of the opacity of the lines into integrals and considering the Kramers equation for the oscillator strengths for level transitions in the hydrogenic atoms as a continuous function of frequency, we can apply the mean value theorem for integrals in order to describe the variation of the global opacity of the lines of all the chemical elements. The high-lying levels in nonhydrogenic atoms as well as their hydrogen-like transitions can also benefit from this method. For the case of stellar atmospheres not in local thermodynamic equilibrium, we can apply our method directly. The compact analytic expressions obtained are easy to use. The application of the method to the hydrogen lines shows the classical line-blanketing effects.