The hydrodynamic numerical modelling package WAMIT is a widely used boundary element method code. It is mainly distributed in its PC executable version which does not include any graphical user interface. Instead, input settings are entered into the program by means of text files. Computation results are output in the same way. Preparation of the input files and post-processing of the outputs is often time consuming and repetitive. This paper presents a graphical user interface addressing this task. The interface, programmed in MatLab, is split into pre- and post-processing parts. Its key feature is its modular structure. The programme consists of numerous independent modules that carry out specific and self contained tasks. This makes it easy to customise the interface to suit new requirements.