With benefits to motivation, reading fluency, learner autonomy and vocabulary knowledge development, extensive reading (ER) is now a common component of university English courses. However, the crucial question of exactly how much ER is required to generate discernible improvement remains largely unanswered. This question is addressed through a year-long study of data on the number of words read by 160 university undergraduates and the gains they made on pre- and post-course vocabulary tests.