<p>An analytical expression for determining the optimal length of the rotor and optimal radius of the rotor of a separation gas centrifuge is obtained. Optimality is defined as the maximum of the separative power per unit volume occupied by the centrifuge with account for the enrichment factor. It is shown that long rotors typical of European centrifuges are not economically justified. The optimal radius is proportional to the value of the centrifuge feed flow.</p>