Visual working memory is constrained by a limited resource that must be shared between multiple stimuli, such that remembering more items decreases the precision with which each item can be recalled. Here we investigated how the sensory quality of an item affects the working memory resource allocated to storing it. In two cued-recall continuous orientation reproduction experiments, we manipulated the signal strength of memoranda either by varying stimulus contrast (internal or sensory noise) or by varying within-stimulus orientation variability (external or stimulus noise). Both manipulations reduced the precision with which the manipulated item was recalled. However, increasing internal noise (reducing contrast) improved recall precision for other items held concurrently in memory, indicating that fewer working memory resources were consumed by the noisy item and freeing capacity for remaining items. By contrast, increasing external noise (within-item variability) impaired recall of the noisy item without benefiting concurrent memoranda. We show that these dissociated effects are predicted by a neural population coding model in which spiking activity acts as the limited working memory resource and reduced sensory drive from low-contrast inputs directly decreases the neural firing rate allocated to storing those items.