A koku (石, koku?) is a unit of volume in Japan, equal to ten cubic shaku. Approximately 3.6 koku equal one cubic metre. The koku was originally defined as quantity of rice, historically defined as enough rice to feed one person for one year. In 1891 it was defined such that one koku equalled exactly 240100/1331 litres, this is approximately 180.39 litres, or about 5 bushels (40 Imperial or 48 US gallons).