The koku (石/石高?) is a Japanese unit of volume, equal to ten cubic shaku. In this definition, 3.5937 koku equal one cubic metre, i.e. 1 koku is approximately 278.3 litres. The koku was originally defined as a quantity of rice, historically defined as enough rice to feed one person for one year (one masu is enough rice to feed a person for one day). A koku of rice weighs about 150 kilograms (23.6 stone or 330 pounds).

In 1891, a smaller koku was defined such that one koku equalled exactly 240100⁄1331 litres, which is approximately 180.39 litres, or about 5 bushels (40 imperial or 48 US gallons).

During the Edo period of Japanese history, each han (fiefdom) had an assessment of its wealth, and the koku was the unit of measurement. The smallest han was 10,000 koku and Kaga han, the largest (other than that of the Shogun), was called the "million-koku domain". (Its holdings totaled around 1,025,000 koku.) Many samurai, including hatamoto, received stipends in koku, while a few received salaries instead. In the Tōhoku and Hokkaidō domains, where rice could not be grown, the economy was still measured in koku but was not adjusted from year to year. Thus some han had larger economies than their koku indicated, which allowed them to fund development projects.

Koku was also used to measure how much a ship could carry when all its loads were rice. Smaller ships carried 50 koku (7.5 t) while the biggest ships carried over 1,000 koku (150 t). The biggest ships were actually larger than military vessels owned by the Shogunate.

In the Meiji period (1868–1912), Japanese units such as the koku were abolished and the metric system was installed.

The Hyakumangoku Matsuri (Million-Koku Festival) in Kanazawa, Japan celebrates the arrival of Lord Maeda Toshiie into the city in 1583, although the Maeda's income was not actually raised to over a million koku until after the Battle of Sekigahara in 1600.

The koku unit is still commonly used in the lumber industry in Japan.