The measurement of the volumes of small (10–100 µm) liquid drops is important in a number of fields including inkjet printing, liquid dispensing and spraying. This article explores the use of synthetic, constructed images, representing shapes with precisely known volumes, and real photographic images of inkjet drops to compare a number of image processing methods designed to estimate drop volume. The synthetic images were generated with a range of sizes, background gray levels and degrees of blur and noise. The image processing methods were chosen to represent a range of approaches, some very simple and some more complex. A comparison of the results from these methods shows that they responded differently to various image features. The process described in this article could be used to compare other existing or new processing methods, and the results should be valuable in the development of standard methods for drop measurement. © 2016 Society for Imaging Science and Technology.