For reproducing processes such as electrochemical machining (ecm) the accuracy of the reproduction is an important process parameter. Characteristic numbers which are a measure of this accuracy are proposed in this paper. Due to the non-linearity of the process a detail transfer function, which is helpful in linear cases, cannot be used, as will be shown from experimental work. Based on the same experiments, a characteristic radius and the standard deviation of a normal profile are proposed to describe the quality of the detail transfer. These numbers, which are easily interpreted criteria for quality and accuracy, have been proved to be almost independent of the geometry. Although developed for optimizing the ecm process, these values are also valid for edm and other reproducing processes. The usefulness of the characteristic numbers is illustrated by a comparison of continuous and pulsed ecm processes.