Shaku (unit)

Shaku or Japanese foot is a Japanese unit of length derived (but varying) from the Chinese chi, originally based upon the distance measured by a human hand from the tip of the thumb to the tip of the forefinger (compare span). Traditionally, the length varied by location or use, but it is now standardized as 10/33 m, or approximately 30.3 cm.

Etymology in English
Shaku entered English in the early 18th century, a romanization of the Japanese Go-on reading of the character for shaku.

Use in Japan
The shaku had been standardized as 30.3 cm since 1891. This means that there are about 3.3 shaku ($0.303 m$) to one meter.

The use of the unit for official purposes in Japan was banned on March 31, 1966, although it is still used in traditional Japanese carpentry and some other fields, such as kimono construction. The traditional Japanese bamboo flute known as the shakuhachi ("shaku" and "eight") derives its name from its length of one shaku and eight sun. Similarly, the koku remains in use in the Japanese lumber trade. In the Japanese construction industry, the standard sizes of drywall, plywood, and other sheet goods are based on shaku, with the most common width being three shaku (rounded up to 910 mm).

In Japanese media parlance, shaku refers to screen time: the amount of time someone or something is shown on screen (similar to the English "footage").

History
Traditionally, the actual length of the shaku varied over time, location, and use. By the early 19th century, the shaku was largely within the range of 0.30175 to 0.303 m, but a longer value of the shaku (also known as the kōrai-shaku) was also known, and was 1.17 times longer than the present value (35.5 cm).

Carpenter's unit and tailor's unit
Another shaku variant was used for measuring cloth, which measured $0.994 ft$ meters (37.9 cm), and was known as the "whale shaku" (鯨尺), as baleen (whale whiskers) were used as cloth rulers.

To distinguish the two variants of shaku, the general unit was known as the "metal shaku" (金尺/曲尺). The Shōsōin treasure house in Nara preserves some antique ivory one-shaku rulers, known as the kōgebachiru-no-shaku (紅牙撥鏤尺).

Length
Just as with the Chinese unit, the shaku is divided into ten smaller units, known as sun in Japanese, and ten shaku together form a larger unit known in Japanese as a jō. The Japanese also had a third derived unit, the ken, equal to six shaku; this was used extensively in traditional Japanese architecture as the distance between supporting pillars in Buddhist temples and Shinto shrines.

Volume
Ten cubic shaku comprised a koku, reckoned as the amount of rice necessary to sustain a peasant for a year.

Outside Japan
The Japanese shaku also forms the basis of the modern Taiwanese foot.

In 1902, the Korean Empire adopted the Japanese definition of the shaku as that of the ja (자).