The relationship between the volume of an intraocular gas bubble and the area of retina covered by the bubble was studied using both a transparent model and a mathematical model of the vitreous cavity. The arc of contact of intraocular bubbles was calculated for vitreous cavities of various diameters. A 0.28-cm3 bubble will cover 90 degrees of retina and be of sufficient size to manage the majority of problems for which an internal retinal tamponade would be useful. Larger retinal tears require disproportionately large increases in bubble volume to achieve modest increases in the area of retina covered. Estimating bubble volume by observing the height of the bubble meniscus in the dilated pupil is subjected to errors induced by small shifts in the angle of observation. A correct evaluation requires that the plan of observation be adjusted so that it coincides with the plane of the meniscus.