Hiro (a unit of length defined by old Japanese system of weights and measures) (尋)

Hiro (尋) is a unit of length defined by old Japanese system of weights and measures.

Originally, it is one of anthropomorphic units defined as the length of adult's outstretched arms. The term 'hiro' and 'hirogeru' (a transitive verb in Japanese, meaning extend) have same etymology. On the other hand, the Chinese characters '尋' is composed of three different Chinese characters: '左' (left), '右' (right) and '寸' (a Japanese old unit of length; 3.03 cm). In measuring the length of something long such as rope, the length is easily represented by the hiro by a manner that a measurer repeats the following: Hold a rope in one's both hands and then outspread the arms. For example, the depth of water is sounded by passing the end of a fishing line down until it reaches to the bottom and then measuring the length of the line.

In the Meiji period, it was provided that 1 hiro equals 6 shaku. Since 1 shaku was fixed at 10 over 33 meters, 1 hiro is equivalent to approximately 1.818 meters. However, the length measured by the hiro varies from person to person, because the hiro is basically derived from the dimension of the human body. Therefore, 1 hiro is sometimes taken as 5 shaku (about 1.515 meters).

Hiro and other units defined by old Japanese system of weights and measures have been still used as a technical term for fishing or in classical rites and festivals, while the Measurement Act prohibits people from using those units for commercial transactions and other purposes at present.

In the English-speaking world, a unit "fathom" with the same definition as the hiro is similarly used to sound the depth of water. Based on the feet system, 1 fathom is also fixed at 6 feet (1.8288 meters).

In China, the units jin (尋), as well as 'jo' (常) that showed double length of jin, were frequently used. These two Chinese characters combined to make a Chinese compound word 'jinjo' (尋常) meaning mediocre and ordinary.