A general method for numerical calculation of power spectra for digital FM signals is developed. Arbitrary baseband pulse shape, the modulation index and the number of levels of the data can be used. The probability density function of the statistically independent data symbols can also be chosen arbitrarily. With this method the autocorrelation function is first calculated and then numerically Fourier transformed, yielding the power spectrum. The time required to calculate a power spectrum on a digital computer is extremely small, and the calculations are simple and easy to use.