Shaku is a unit of length in the traditional Japanese system of measurement known as shakkanho. Shakkanho was established under the Weights and Measures Act of 1891. Under this law, “shaku” was defined as the unit of length, “sho” as the unit of volume and “kan” as the unit of mass. This continued to be used as the Japanese system of weights and measures until 1959 when, after several phases of transition, the units were standardized under the metric system.

Daishaku (large shaku) and shoshaku (small shaku) were established under the Taiho Code in the year 701 and by the Edo era, various types of shaku came into existence. For example, Shogun Tokugawa Yoshimune introduced a unit called kyoho-shaku and there was also a unit called matashiro-shaku. Ino Tadataka used a unit called setchu-shaku when he went surveying across the country to produce a map of Japan. This is said to have been a unit obtained by averaging kyoho-shaku and matashiro-shaku. In present-day Japan, the unit of length has been fully standardized under the metric system. However, the use of two types of shaku, kanejaku and kujirajaku, are permitted to this day. Kanejaku was the basic unit of the shakkanho. Under the Weights and Measures Act of 1891, 1 shaku was defined as 10/33m (approx. 30.30cm). This is used today in the construction of Japanese-style houses. Kujirajaku, on the other hand, is 1.25 times longer than the kanejaku (1 shaku = approx. 37.88cm) and is used in Japanese dressmaking.

The words shaku, sun (a tenth of a shaku) and bu (a tenth of a sun) are still used frequently in the Japanese language to represent the concept of length. For example, when something does not have adequate length, this is often described as being “a sun (or a shaku) short”, and there are also expressions using these units to describe situations where there is not even the slightest flaw or opening, etc. Similarly, the Japanese words for “reduced scale” and “actual size” used in blueprints contain the characters “shaku” and “sun”. Moreover, the units are also used for the sets and props on the kabuki stage and for timber measurements. For example, the standard size of a plywood board is 3 shaku by 6 shaku (sanroku size).