The aim of this paper is to present a novel, universal, methodology for the design of low cost tubular digesters. This method improves on the established methodology by avoiding assumptions that tend to reduce the final hydraulic retention time (HRT) of digesters once installed. This work recommends designing the digester using trench cross-sectional area and proposes an optimization of the trench dimensions with respect to the angle of the walls and the relationship between the length of the biogas bell and the top width of the trench. The influence of the biogas pressure is considered. A simple geometrical analysis is presented that, by parameterization, can be applied in a wide range of situations.