Cosmic-ray soft errors from ground level to aircraft flight altitudes are caused mainly by neutrons. We derived an empirical model for estimation of soft error rate (SER). Test circuits were fabricated in a standard 0.6-µm CMOS process. The neutron SER dependence on the critical charge and supply voltage was measured. Time constants of the noise current were extracted from the measurements and compared with device simulations in three dimensions. The empirical model was calibrated and verified by independent SER measurements. The model is capable of predicting cosmic-ray neutron SER of any circuit manufactured in the same process as the test circuits. We predicted SER of a static memory cell.