De Biagi et al. (2017) have proposed a procedure for building a fallen block volume-frequency law for rockfall phenomenon. The input data are got both from the recording of rockfall events and from the survey of fallen block volumes. The epistemic uncertainty present in the approach affects the value of the parameters of the law. It is shown how to quantify the error due to missed events and to limited set of measured blocks. A procedure for getting a rock block volume for the design of engineering applications is proposed.