1 tan was determined as 360 bu as the area which can produce enough rice to feed 1 person in 1 year under the Taiho Code. It was calculated by multiplying 1 bu (see page 112) by 360 days in 1 year of the old calendar. Then, it was revised to 300 bu in line with agricultural technological progress due to Toyotomi Hideyoshi's nationwide land survey (1582 CE).