Space in the inferior conjunctival sac has not been studied in sufficient detail to allow optimal physical design of conjunctival inserts. We analyzed shape and volume of partially expanded, anesthetized, left inferior conjunctival sacs in 10 young adult subjects by injection of a liquid polysiloxane molding compound. Mean volume, greatest thickness, central thickness, horizontal width, and vertical height of 60 molds were 125.7 microliters (SD = 55), 1.56 mm (SD = 0.69), 1.46 mm (SD = 0.62), 20.7 mm (SD = 2.2), and 8.9 mm (SD = 0.8), respectively. Volumetric and linear dimensions varied between subjects, but certain features were common to all molds: 1) a crescent shape horizontally; 2) a thick inferior horizontal ridge; and 3) a wedge-like shape sagittally. We postulate several advantages of conjunctival inserts with features representative of our molds of conjunctival sacs, e.g. such inserts may be more comfortable, less expelled, larger in volume, and contact more tissue area.