Symbionts can provide hosts with effective protection from natural enemies, but it can sometimes come at a cost. It is unclear to what extent the density of symbionts modulates the cost and benefits of conferred protection. Here we use a meta-analysis of 103 effect sizes from a broad taxonomic range of protective symbioses, to show that the degree of both protection and cost afforded to hosts is a positive function of symbiont density. We found that the effects of symbiont density on protection and cost are robust across ecological contexts. Density-function relationships did not vary with host type, symbiont localization or transmission mode, nor the method of density manipulation. Together, our results suggest symbiont density can be a key variable determining the costs and benefits of a protective interaction.