Estimating the shape and size of a scanning tunneling microscope (STM) tip before scanning is often necessary for the correct interpretation of the STM data. This is particularly essential when using the STM as a metrology tool. It is common among researchers to idealize the tip shape as conical, spherical, or parabolic. By using a field ion microscope (FIM), the tip shape and size can be measured directly on an atomic scale. Based on FIM measurements, it has been realized that simple idealizations of the tip shape are not satisfactory in many cases, particularly for sharp tips. In the present work, we have developed an enhanced model of the shape of a STM tip measured with FIM using Lame's curves. The proposed model, which assumes an axisymmetric tip shape, is validated with comparisons between the theoretical simulations of FIM images and experimental FIM images from tips made of tungsten (110). For tips smaller than a limiting size, applications of the model may no longer be valid. (C) 2004 American Vacuum. Society.