According to the tractor theory, the operating mass of a tractor is calculated from its traction force required to overcome the resistance of soil-cultivating implements and provisional incidental overloads. Implements' resistance changes are estimated by the coefficient of possible overload -- the ratio of maximum possible resistance to its average value, i. e. the maximum resistance value of the implement is used for calculations One of the regulating agricultural operations in the choice of tractor's operating mass is plowing. Besides, the implement's resistance change with the speed increase of the tractor's aggregate is not taken into account. The operating tractor's mass calculated by this method is by 10…20% different from the optimum operating mass.With the aim to bring the calculated values of tractor's operating mass to the optimum ones, we suggest analytical dependencies intended for determining the optimum operating masses of tractors for the given nominal hook pulling force values, and for determining the nominal hook pulling force values for the known values of the optimum operating mass. The expressions obtained take into account the implements' resistances increase for higher speeds of the tractor aggregate. The optimum operating tractors' masses calculated by means of the analytical expression suggested yield good matches with the real values. It resulted in a nomograph for determining the plow resistance depending on the plow body width and the number of bodies in the plow, as well as the optimum operating mass of the tractor intended to work with the plow. The nomograph may be used by design companies for new tractor models, and by farmers during use of tractor aggregates, as well as to make a proper choice of tractor machinery for purchase.