In the U.S., there are numerous ongoing efforts to remedy the word gap: massive differences in heard vocabulary for poor versus advantaged children during the first 5 years of life. One potentially important resource for vocabulary exposure is children's book reading sessions, which are more lexically diverse than standard caregiver-child conversations and have demonstrated significant correlational and causal influences on children's vocabulary development. Yet, nationally representative data suggest that around 25% of caregivers never read with their children. This study uses data from 60 commonly read children's books to estimate the number of words that children are exposed to during book reading sessions. Results showed that parents who read 1 picture book with their children every day provide their children with exposure to an estimated 78,000 words each a year. Cumulatively, over the 5 years before kindergarten entry, researchers estimate that children from literacy-rich homes hear a cumulative 1.4 million more words during storybook reading than children who are never read to. These results suggest that home-based shared book reading represents an important resource for closing the word gap.