The widespread use of molecular markers to estimate parentage makes possible a new index of the opportunity for sexual selection. After demonstrating the need for a new measure, I develop one based on the upper limit on sexual selection. I describe what sets the upper limit for each sex by showing how maximum fecundity increases with number of mates, accounting for the amount of energy (or critical resources) available for reproduction and levels of parental care. For females the upper limit on sexual selection is set by the value of paternal investment that comes with each mating. For males, the upper limit on sexual selection is set by the fecundity of their mates (including any boost to female fecundity from paternal investment). Sex-roles are most likely to reverse (making males choosy and females competitive) when the amount of reproductive energy investment made by each sex is low, irrespective of the level of paternal investment. Finally, I propose that we use the difference between male and female upper limits on sexual selection to quantify sex differences in the opportunity for sexual selection. Using upper limits to estimate the opportunity for sexual selection is more intuitive than older methods (e.g., standardized variance in mating success), it is experimentally measurable, and it is valuable in understanding the evolution of mating systems.