This paper describes a numerical method, by using FFT and IFFT, for computation of an output response impulse waveform of a high voltage divider that is characterized by a step response. Comparing with convolution integration, the method can be easily implemented and used for evaluation of a high voltage divider. By comparison of the difference between the input forcing impulse and output response impulse, an uncertainty bound on the high voltage divider can thereby be estimated. It is suggested that the error caused by the high voltage divider can be regarded as an offset error that must be linearly added to the expanded uncertainty.