An approximate analytical expression for the rate of loss of cloud droplets by coalescence in warm clouds is derived from the stochastic collection equation (SCE). The expression depends only upon precipitation rate and cloud droplet concentration and compares well with estimated loss rates derived using observed cloud drop size distributions and the complete collection kernel. Loss rates are found to be surprisingly high even for the modest precipitation rates found in drizzling boundary layer clouds and can be used to infer the loss rate of cloud condensation nuclei (CCN) through coalescence. The expression can be used to better represent the interdependence of aerosol and cloud properties in the boundary layer.