In the work the physical and mathematical models of the process of arc welding by powder wire are given. Are examined partial conversion factors relative to welding by powder wire, the procedure of their estimation, and also the interrelation of partial and averaged of conversion factors with the calculation of weld metal. With the welding by the powder wire the composition of weld metal and its technological welding properties depend on the conditions tape and the charge and interaction of the formed phases with each other and gas. The composition of weld metal can be determined, if the general, average conversion factors of elements are known. The latter find most frequently experimentally for the specific conditions for welding, that does not make it possible to guarantee the correctness of forecast with the changed conditions. In connection with this, was set the task of developing the procedure of the determination of the partial conversion factors of elements, i.e., conversion factors at each stage of heating and melting, and their dependence on the parameters of welding conditions. It shows the common conversion factor of element, what share of element is lost in the process to welding. The analysis of the processes, which take place during heating and melting the powder wire, made it possible to find the interrelation of general (average) conversion factor with the partial coefficients, expressed quantitatively. Are given the equations, which will allowed on the basis of experimental and reference data to find the partial conversion factors of elements.