For the first time, 3D multiphysics finite element modelling has been used to optimise the geometry of a calorimetric thermal flow sensor. The model involves and couples three physics domains: electric, thermal and fluid mechanics. The model is validated against experimental data obtained from a thermoelectronic flow sensor comprising of a tungsten heating resistor and temperature sensing diodes. Upstream and downstream diodes measure the temperature change caused by the asymmetric thermal profile when gas flow is introduced. The optimum distance between the diodes and the heater is shown along with the advantages of altering heater and membrane geometries, providing the knowledge for application-driven sensor optimisation.