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The Shaku (尺) is a traditional Japanese unit of length, forming a fundamental part of the traditional Japanese system of weights and measures known as the Shakkanhō (尺貫法). It is roughly equivalent to the English foot, although slightly shorter in its modern standardized form.

The Shaku is traditionally subdivided and multiplied decimally:

• 1 Jō (丈) = 10 Shaku
• 1 Shaku (尺) = 10 Sun (寸 - Japanese inch)
• 1 Sun (寸) = 10 Bu (分 - Japanese line)

There were historically different types of shaku, most notably the standard kanejaku (曲尺) used in carpentry and the longer kujirajaku (鯨尺) used for measuring cloth. Unless otherwise specified, "shaku" usually refers to the kanejaku.

The Shaku originated from the Chinese Chi (尺), introduced to Japan likely between the 3rd and 6th centuries AD. Like its Chinese counterpart, the length of the Shaku varied considerably across different historical periods and regions in Japan.

• Early Adoption: Early Japanese measurements were heavily influenced by Chinese standards, but the length of the Shaku began to diverge over time.
• Edo Period (1603–1868): Different standards coexisted, including the kanejaku (carpenter's shaku) and the kujirajaku (cloth shaku, literally "whale shaku," possibly referring to rulers made from whale baleen). The kanejaku became the more common standard for general measurement.
• Meiji Restoration (1868 onwards): Japan began modernizing and standardizing its systems. In 1891, the Weights and Measures Act defined the standard Shaku (kanejaku) legally in relation to the metric system as exactly 10/33 of a meter. This aimed to provide a precise conversion factor while preserving the traditional unit. The kujirajaku was defined as 1.25 times the kanejaku (i.e., 12.5/33 of a meter).
• Metrication: Japan officially adopted the metric system in 1951, and its use became mandatory for most official and commercial transactions by 1966. However, the Shaku and other Shakkanhō units remain in use in specific traditional fields.

While the metric system is standard in Japan, the Shaku persists in several areas:

• Traditional Japanese Architecture and Carpentry: The kanejaku is still widely used for measurements in building temples, shrines, traditional houses, and making furniture. Architectural modules like the ken (間) are often based on the Shaku. Japanese woodworking tools are often sized or marked in Shaku and Sun.
• Kimono and Traditional Clothing: The kujirajaku is the standard unit for measuring cloth width (tanmono) and dimensions when making kimonos and yukatas.
• Traditional Crafts: Used in making items like tatami mats, shōji screens, and other traditional crafts where historical dimensions are important.
• Musical Instruments: The length of traditional Japanese instruments, notably the shakuhachi (尺八) flute, is measured in Shaku and Sun (its name literally means "1.8 shaku").
• Martial Arts: Some traditional martial arts (koryū budō) may use Shaku-based measurements for weapon lengths (e.g., jō, bō) or stance dimensions.
• Gardening and Landscaping: Traditional Japanese garden design sometimes employs Shaku-based proportions.
• Historical and Cultural Context: Essential for understanding dimensions in historical texts, art, and artifacts.