Ri (old Japanese unit of length, approximately 3.927 km) (里)

Ri is a unit of length in old Japanese system of weights and measures. It is equivalent to about 4 kilometers in today's Japan, 500 meters in China and about 400 meters in Korea.

Originally it was a unit of length during the Zhou period in ancient China. 1 ri represented the area of 1800 shaku (unit of length, approximately 30.3 centimeters) square (1800 shaku is equivalent to 360 bu [unit of length in old Japanese system of weights and measures] or 6 cho [unit of length, approximately 109 meters]).
Later, the length of its one side became the unit of length 'ri.'
1800 shaku is equivalent to 540 meters when one shaku is 30 centimeters. Afterward, although changes occurred through periods, the length of ri has returned to that in the Zhou period because 500 meters has been set as 1 ri in today's China.

Ri' came down to Japan in those days and 1 ri was set as 5 cho in the Ritsuryo System (a system of centralized government based on the ritsuryo code). Various kinds of ri were used by periods and regions after the collapse of the Ritsuryo System. They were about between 5 and 6 cho.

However, it is difficult to measure the length of 'ri' directly because it is long distance. Therefore, the distance walked in the time to be spent roughly to walk 1 ri was called 1 ri. There appeared various length of ri (36 chori, 40 chori, 48 chori and so on) because people's pace of walking was different according to topography and road conditions. But this definition had an advantage that people could find the time required only by the number of ri to the destination. However, as this would cause confusion, 36 chori was fixed as the standard ri in the Edo period with accepting the existence of various ri. In the Meiji period, 1 ri was fixed to be equal to 36 cho and other kinds of ri were banned by The Weights and Measures Act which was established in 1891 after joining the treaty of the meter. 1 ri is equal to the product of 36 cho multiplied by 360 (shaku per cho) and then 12960 shaku, because 1 cho is equal to the product of 60 ken (a unit of length) multiplied by 6 (shaku per ken) and then 360 shaku. 1 ri is equal to the product of 12960 shaku multiplied by (10 over 33) (meter per shaku) and then (43.2 over 11) kilometers and then 3.9 kilometers (the part of overbar is a recurring decimal) because 1 shaku has been fixed as (10 over 33) meters.

There was a period when Chinese ri was called "shinari" (Chinese ri, approximately 500 meters) in Japanese literature for distinction because 1 ri in Japan and in China had quite different length as mentioned above.

1 ri was set as (a half) kilometers, that is, 500 meters and the letter of '里' (ri) has also been used for 'kilometer' in China. The former is called li while the latter is called gongli for distinction. 1 li is equal to 1500 shi-chi (1 over 3 meters, or approximately 33.3 centimeters) (see also the article of Shizhi [Chinese units system of measurement]).