The article presents the results of research and experimental studies aimed at measurement of crack width in concrete under the impact of mechanical loading. During the research, the author simultaneously measured load, deformation, crack width, acoustic emission gross counting by tensiometric and acoustic methods and obtained full diagrams of concrete deformation. The experiments resulted in regression equations and their combined diagrams showing relation between acoustic emission gross counting and mechanical loading level. The author identified the parameters of macro-crack initiation and their width in lightweight concrete depending on the value of the applied load. The conducted experiments and statistic processing of the data produced the correlation between the crack width and acoustic emission gross counting.