When one or more sinusoidal waves pass through a nonlinear device, the output contains harmonic and product tones in addition to tones of the same frequencies as those in the input. A method is presented whereby the amplitudes of distortion products of various forms can be quickly and easily obtained from single-tone characteristics. Certain numerical factors have to be computed beforehand, but once these are known the method is immediately applicable to any shape of characteristic.The numerical factors have been evaluated for a number of different distortion mechanisms. Predicted distortion levels are found to agree well with measured values. One example considered is that of multicarrier operation of travelling-wave tubes, which has been analysed in detail in an earlier paper. it appears that a substantial saving in cost and elaboration of test equipment would result from the use of the present approach, rather than the test procedure recommended previously.