The goal of this presentation is to give some information about the sizing of electrostatic lenses, mainly the focal length. These lenses are used to focus particles beams. It is proposed a small program and formulas taking account different parameters (voltages and configuration) in a relative simple way. A “freeware” program can, possibly, help the reader to more precisely design lenses. This presentation relies on a personal simulator. A physical explanation of the focusing principle is proposed. It is afterwards explained why a negative potential will also focus an ions beam, even it seems counter-intuitive. It is also shown that the quality of convergence increases with the thickness of the lens. Moreover, it is described the expected behavior of lenses in presence of a strong space charge, or in presence of two different types of plasma (hot ions/cold electrons plasma and fusion neutral plasma).