Flexible-resource theories characterize working memory as a flexible resource that can store either a large number of low-quality representations or a small number of high-quality representations. In contrast, limited-item theories propose that the number of items that can be stored in working memory is strictly limited and cannot be increased by decreasing the quality of the representations. We tested these fundamentally different conceptualizations of working memory capacity by determining whether observers could trade quality for quantity in working memory when given incentives to do so. We found no evidence that observers could increase the number of representations by decreasing their quality in working memory, but observers could make such a trade-off at earlier processing stages. Our results show that the capacity limit of working memory is best characterized as a limit on the number of items that can be stored and not as a limit on a finely divisible resource that simultaneously determines the number and quality of the representations.