We calculate Keplerian (mass shedding) configurations of rigidly rotating neutron stars and quark stars with crusts. We check the validity of empirical formula for Keplerian frequency, f_K, proposed by Lattimer & Prakash, f_K(M)=C (M/M_sun)^1/2 (R/10
km)^-3/2, where M is the (gravitational) mass of Keplerian configuration, R is the (circumferential) radius of the non-rotating configuration of the same gravitational mass, and C = 1.04 kHz. Numerical calculations are performed using precise 2-D codes based on the multi-domain spectral methods. We use a representative set of equations of state (EOSs) of neutron stars and quark stars. We show that the empirical formula for f_K(M) holds within a few percent for neutron stars with realistic EOSs, provided 0.5 M_sun < M < 0.9 M_max,stat, where M_max,stat is the maximum allowable mass of non-rotating neutron stars for an EOS, and C=C_NS=1.08 kHz. Similar precision is obtained for quark stars with 0.5 M_sun < M < 0.9 M_max,stat. For maximal crust masses we obtain C_QS = 1.15 kHz, and the value of C_QS is not very sensitive to the crust mass. All our Cs are significantly larger than the analytic value from the relativistic Roche model, C_Roche = 1.00 kHz. For 0.5 M_sun < M < 0.9 M_max,stat, the equatorial radius of Keplerian configuration of mass M, R_K(M), is, to a very good approximation, proportional to the radius of the non-rotating star of the same mass, R_K(M) = aR(M), with a_NS approx a_QS approx 1.44. The value of a_QS is very weakly dependent on the mass of the crust of the quark star. Both as are smaller than the analytic value a_Roche = 1.5 from the relativistic Roche model.
