Flow is a mental state in which an individual becomes immersed in an activity as a result of an optimum balance of interest, control, and challenge. Such a flow state overlaps with descriptions of ideal extensive reading experiences eg. becoming engrossed in a story and losing track of time. At the same time, teachers often assign reading targets using a variety of methods including book counts, word counts, and reading time in order to encourage students to read more. These different measurements of reading quantity may influence students' choice of reading materials as well as their reading experiences. This presentation compares student book choice behavior and experiences of flow while reading under the differing conditions of reading assignments given in terms of weekly word count goals with those of weekly reading time goals. Student reading records from the online reading library Xreading are used to compare differences in book choice behavior, such as book length and difficulty level, while experiences of flow are compared using mean flow scores from weekly questionnaires and independent measures t-test between the word count and reading time groups.