“Tsubo” is about 6 square shaku which is equivalent to 2 sheets of tatami mat. I reckon more people in Japan are familiar with this measurement for land and building area than they are with square meters. In the period of the Taiho Code (701 CE), it was called “bu” and it was apparently seen as the area it takes to produce enough rice to feed one Japanese person in one day, that is to say, 3 go (1 go ≒ 180 mL) of rice.