The relationship between the generated power and the mode volume of the fundamental mode of a plane-sphere resonator is considered in the article exemplified a He-Ne laser. A novel technique based on the volume of the generating mode has been proposed to estimate the output radiation power of a gas-discharge laser with an arbitrary cross-sectional shape of an active element. It was shown that the results of calculations of the generated power, carried out on the basis of the effective mode volume, which took into account the transverse distribution of the population inversion of the active medium, were in better agreement with the experimental data and the results of estimates using other methods.