This work consists in the analytical derivation and numerical solution of the equation which determines the shape of the section of a silo bag: a long horizontal cylindrical plastic bag filled with either a liquid or a granular material which behaves similarly, exerting normal, but not frictional forces against the wall. The bag is considered inextensible and completely flexible, capable of supporting only tensile loads. These suppositions lead to a secondorder differential equation for the membrane shape, which is normalised and solved, in a way that allows, for any bag with any amount of filling, a simple computation of its geometry, enclosed area and tensile loads. A discussion is included about the effects on the theoretical results of the silo bag tensile deformation.Grupo Fluidodinámica Computacional