A technique for calculating the flowrate of cryogenic two-phase flows in separationless flowmeters based on a venturi is proposed. The technique takes into account the equation of state of the two-phase medium, that is changes in density, mass vapour quality and temperature of cryogens two-phase flows against pressure in the venturi. It is shown that without an accounting of the flow parameters evolution, an additional flowrate error appears and therefore it is impossible to correctly determine the sensitivity and measurement range of the flowmeter. The most optimal measurement range of the pressure drop is chosen as well as a method of the flowrate measuring by the temperature difference across the venturi is proposed. The problem of determining the flowrate of two-phase cryogenic flows is topical for accelerator, aerospace and gas-transport engineering.