With only about a third of students in US public schools achieving the NAEP Proficient level, many educators believe students are not getting enough reading practice. However, how much reading practice is enough? This study quantifies the relationship between the amount of book reading and the expected number of words learned. We collected 45 children's novels, M(Lexile) = 994 L, or Grade 6, and assembled a corpus of 3.2 million words. When reading a book from the corpus, sixth graders would on average encounter over 1000 word types above their grade level. We estimated that they could learn about 300 of these words, based on word repetitions and literature on incidental word learning. Through simulation, we demonstrated that sixth graders would need to read half a million words from the corpus to learn 2000 words incidentally. The implications for using book reading to promote vocabulary growth are discussed.