The paper presents an approach for calculation of a kinetic high vacuum pump pumping performance for molecular mode of a gas flow. The calculation is based on the general assumption for gas molecular-kinetic theory: thermal motion velocity molecular distribution is described by Maxwellian distribution, gas molecules interaction is absent, the interaction of a solid body surface and gas molecules is described in terms of the diffuse reflection law, the accommodation coefficient equals unity. Direct statistical modeling for calculations of flow parts pumping features and analytic dependence connecting their joint operation enabled sufficiently to reduce calculation time with no decrease in calculation accuracy and with no distortion of pumping physics. The suggested mathematical relations allow to calculate present kinetic high vacuum pumps (turbomolecular, molecular and combined turbomolecular with molecular stages) considering the effect of a suction branch and overflows in pump flow parts.