The effusion rate of lava from a volcanic vent is an important quantity controlling the length of the ensuing lava flow. The ability to evaluate the effusion rate is therefore of importance to estimate the hazard connected with the flow. In the present paper we provide formulae giving the effusion rate and the lava viscosity for two- and three-dimensional (3-D) Newtonian and Bingham flows. In order to keep the difference between the actual lava effusion rate and the model estimation within 10%, the 3-D model must be used if the ratio between the width and the thickness of the lava flow does not exceed the value 13. The Bingham model must be used if the ratio between the plug thickness and the thickness of the lava flow exceeds the value 0.07. An analogous consideration can be made for viscosity.