This study presents a model to evaluate the storage capacity of alternative fuels for displacement vessels. The model utilises vessel particulars and geometric parameters to calculate the available storage space and energy-carrying capacity for various tank configurations while ensuring compliance with geometric and regulatory constraints. Validation was conducted using bulk carriers of varying sizes using liquid hydrogen fuel, demonstrating a strong linear correlation between vessel deadweight and storage capacity. The analysis identified both lower and upper limits of maximum storage under realistic operational scenarios, highlighting significant variability based on design constraints. These findings underscore the importance of optimising ship design to mitigate constraints and maximise storage capacity, pushing it towards the upper limit to enhance operational efficiency.