Research on sperm is incorporated into many areas of ecology and evolution including sexual selection, reproductive physiology and ecotoxicology, as well as comparative studies in evolution and phylogenetics. Currently, producing data on sperm morphology involves several time-consuming steps, particularly photographing sperm and measuring their length (e.g. head, midpiece, tail and total sperm length). Here, we present Sperm Sizer, a freely available Java program that semi-automates the process of measuring sperm length along the centre of the sperm (including head, midpiece, tail and total length). We compare sperm measurements made with Sperm Sizer to those made with the widely used non-automated software ImageJ, for sperm from a single bird species (the long-tailed finch Poephila acuticauda), eight species of passerine bird and eight species of lizard, and provide examples demonstrating that the program can measure at least some mammalian, fish and mollusc sperm. Sperm length measurements from Sperm Sizer are highly correlated to those made using ImageJ, demonstrating that Sperm Sizer produces high quality sperm length data while taking drastically less time. Our data suggests that Sperm Sizer measurements could possibly be incorporated into existing large datasets with a small correction, although this will need to be assessed on a case-by-case basis. We suggest that generally, sperm image quality (high contrast, minimal overlap of sperm, etc.) will be more important than the shape of the sperm for whether or not Sperm Sizer can be employed for a given project.