Several indirect methods have been proposed to measure bed-load transport rates. We analyze the relationships between bed-load transport rates, water discharge, and sensor response and determine what can and what cannot be measured with such sensors. We argue that indirect sensors are in most cases unsuitable to develop a rating curve of bed-load transport rates. Instead, they can be used to estimate sediment yields of individual events as proposed previously. We develop a method to estimate the standard deviations of transport rates and apply it to the field data from the Pitzbach in Austria, where direct high-resolution bed-load transport measurements are available. The method gives good results, depending on the choice of bed-load function used for the calculation. The standard deviations are overestimated by about ~30%.