Application of two-parameter Rosin-Rammler approximation in sedimentological analysis is considered. Normalized weight of polydisperse sediment is approximated as a function of time. It is believed that time of precipitation of particles depends only on dynamic viscosity of liquid, height of vessel, radius of particle, density of particle, density of liquid, acceleration of gravity. In this case, it is assumed that mentioned values are constant. To find approximation parameters, method of least squares and nonlinear programming are used. Differential curve is constructed and position of its maximum is located. Computations are realized in VBA language in SEDA.xls.