Working memory is an often studied and important psychological construct. The growth of working memory capacity (WMC) in childhood is described as linear. Average adult WMC is estimated as either four or five "chunks." Using latent curve models of data from a measure of digit span backward that was administered longitudinally to a large sample representative of the native-English-speaking U.S. kindergarten population in 2011, we found that WMC growth in childhood is curvilinear. It shows an increasing yet decelerating pattern. Scoring rules (e.g., requiring 50% or 75% of trials correct) influence age-based estimates, but WMCs have likely been underestimated in children, and the average adult WMC of five is more plausible than four, as measured by digit span backward. Developmental WMC estimates, such as those reported in this research, may help others develop prescriptive learning interventions for children and understand its growth and decline across the life span.