The Centre for Industrial Diagnostics and Fluid Dynamics (CDIF UPC) has collaborated with Roca to validate a model of fluid dynamics simulation for toilets. The project had two objectives. The first was to demonstrate that simulation is a valid model with results similar to those obtained by experimentation. The second was to use a toilet that is already on the market to analyse and compare alterative siphon geometries that would improve the features of a standard toilet, using previously defined parameters (biphasic behaviour of the flow -water and air-, depression variations, jet flow volume, turbulence model, length of the process, among others). This numerical simulation model leads to considerable cost savings in the siphon design stage.