Among the main components of gearboxes is the housing, which hold the other gearbox’s components and supports and dissipates the loads generate between them. However, despite its importance, there are no clear procedures for its design, not even specific computational tools, which could improve the efficiency of the design process. This article describes the development of mathematical models that permit to determine the highest stress in welded housings of gearboxes with cylindrical gears and horizontal parallel shafts. The models are used to complete the housing dimensioning. To reach this objective, several parametrical analyses were developed by the authors, considering the internal housing dimensions and the bearing loads. It was observed that the zone below the bearing of the output shaft presents the highest stresses. Initially the stress in this zone was estimated by means of analytical methods, modeling it as a one end fixed column with combined load in the other end. To correct the results, numerical simulations of the whole housing were made, considering the effect of the housing width and the distance between the two last bearings. Using the developed models, it is possible to find the lateral wall thickness of the housing, considering commercial plate thickness, with a safety factor between two and three.