Fertilisation and development in broadcast-spawning marine invertebrates depends on the ability of the egg to attract spermatozoa and provision progeny. Echinoderm eggs have a jelly coat that facilitates sperm–egg collisions. We investigated variation in egg volume and target area for spermatozoa provided by the jelly coat within and between three sea urchin species (Heliocidaris erythrogramma, Heliocidaris tuberculata, Centrostephanus rodgersii), as well as across 22 echinoderm species for which data are available. Egg and jelly-coat size varied within spawns of individual females, between females of a species and between species. The jelly coat increased egg target area by 125–489% for echinoids with planktotrophic development. In general, planktotrophic echinoids (n=16) with larger eggs had thicker jelly coats, as did H. tuberculata females with larger eggs. Variability in egg and jelly-coat size within a species indicates that these traits are prone to maternal effects and may be influenced by factors such as sperm environment and offspring fitness that drive selection on egg investment. The greater variability in jelly-coat size compared with that of the egg has a large potential to affect fertilisation and should be considered in models of fertilisation kinetics. Egg size alone cannot be used as a metric to infer target size for spermatozoa.