Building design and grounds might contribute to physical activity. The size of school grounds, and in-school physical activity were compared of 1566 sixth-grade girls. On average, the school footprint size accounted for 4% of all light physical activity and 16% of all moderate-to-vigorous physical activity during school hours. School design appears to be associated with physical activity, but it is likely that programming (eg, physical education, intramurals, club sports), social factors, and school siting are more important determinants of total physical activity.