Ocular magnetic resonance imaging, enabling 3D tumour imaging and providing better soft tissue contrast than conventional ultrasound, is increasingly used for uveal melanoma. The aim of this research was to determine the difference in geometrical tumour measurements between ultrasound and MRI. For this purpose, tumour prominence and largest basal diameter were calculated automatically based on MR imaging and compared to the US measurements. Differences >0.5mm were observed in 55% of prominence measurements and 80% of LBD measurements. Furthermore, automatically measuring tumour geometry based on 3D MR imaging lead to the discovery of inconsistencies in measurement definitions within and across disciplines.