We report a study of the aggregation of silylated glass spheres at the water–air interface. The particle size (75-μm diameter) was selected so that the aggregation could be recorded in situ by a digital camera. Numerous relevant parameters of the aggregation can be expressed as functionals of the cluster size distribution function (e.g. size and number of clusters, polydispersity). Our goal was to find links between these functionals and measurable quantities such as the surface coverage of the aggregation area. We compared the aggregation of the systems of various starting conditions through the aforementioned functionals in real experiments and computer simulations.