The dependence of droplet concentration on the vertical velocity at the cloud base of stratiform and cumulus clouds and aerosol size distribution is calculated. A simple parameterization of droplet concentration under different aerosol conditions is described. The parameterization is represented in two ways: (1) by a regression equation that expresses the dependence of droplet concentration on the concentration of cloud nuclei (CN), on the parameters of the CN size distribution and on the vertical velocity at the cloud base and (2) by look-up tables. The wide range of parameter variations was separated into different subranges typical of maritime, continental, and extreme continental clouds, both cumulus and stratiform. For each region a particular regression equation was derived. The coefficients in the equation were determined using the optimum experiment planning theory. In the simulations a 2000-bin microphysical parcel model was used. It was found that among all parameters tested the aerosol concentration is the dominating factor. The comparison of the droplet concentrations obtained using the regression formula with the droplet concentration observed indicates a good agreement within a wide range of conditions. The analysis of the dependence of droplet concentration on the microphysical and dynamical parameters within each region makes it possible to reduce the procedure of droplet concentration calculation to a simple interpolation of the concentrations calculated in the center of each region. These look-up tables are presented. All CN are assumed to be soluble. In case some fraction of CN is insoluble, a simple correction of the CN concentration should be performed. Parameterization can be used in different numerical mesoscale and large-scale models whose resolution does not allow one to reproduce vertical velocities at the cloud base and supersaturation maxima explicitly. The results can be also useful in remote sensing studies.