The present invention relates to a method and apparatus (5) for determining a change in volume within a vessel (11) with regard to fluid (14) being added or removed from the vessel. The method involves measuring first and second surface levels (20, 20′) of fluid (14) within the vessel (11) (with regard to fluid being added or removed therefrom), first and second roll values (23, 23′) of the vessel, and first and second pitch values (26, 26′) of the vessel. From these measurements, and in conjunction with the known interior dimensions and known interior total volume of the vessel, there are calculated first and second fluid volumes that are compared to determine a change in fluid within the vessel.