The objective of this paper is to empirically evaluate OOmFPWeb, a functional size measurement procedure for Web applications. We analyzed four data sets from a family of experiments conducted in Spain, Argentina and Austria. Results showed that OOmFPWeb is efficient when compared to current industry practices. OOmFPWeb produced reproducible functional size measurements and was perceived as easy to use and useful by the study participants, who also expressed their intention to use OOmFPWeb in the future. The analysis further supports the validity and reliability of the technology acceptance model (TAM)-based evaluation instrument used in the study.