Shakkanho (traditional system of weights and measures) (尺貫法)

Shakkanho is one of unit systems for measuring length, area, and so on. It is widely used in East Asia. The term "Shakkanho" represents two basic units, shaku for measuring length and kan for measuring mass. However, 'kan' is a unit only used in Japan, and therefore the term of Shakkanho is also used only in Japan. In the narrow sense, Shakkanho exclusively refers to the unit system indigenous to Japan. The Chinese unit system corresponding to Shakkanho is Shizhi, because the unit indigenous to China is not kan but kin. In this section, the unit system originated in China, which was used or is used throughout the East Asia region, is described as Shakkanho in the wide sense. Meanwhile, in Japan today, the Measurement Act prohibits using Shakkanho in transactions and certification, imposing a penalty of 500,000 yen or less on an offender. However, practically, such an offense is connived in the traditional sectors.

Summary
Shakkanho originated in China. Like in the yard-pound system in the West, the dimensions of the human body parts, the mass of grain, and so on, had been used as units. However, the unit system was specifically defined by degrees. A supreme example is the definition based on the Oshiki Kyosho theory described in the Santo-reki calendar (astronomical system), which was proposed by Liu Xin in the end of Former Han Dynasty (China). As for length, the theory defined the diameter of a grain of black millet as 1 bu (分; 0.3 centimeters or 0.1 sun), while the length of an ancient flute called Kosho, a standard instrument to fix the chromatic scale, as 90 bu (9 sun [a unit of length]). Meanwhile, the volume of Kosho (810 cubic bu) was defined as 1 yaku (0.5 go [a unit of mass]), and the mass of 1,200 grains of black mille, which could fill up the inside of Kosho, was defined as 12 shu (0.5 ryo [a unit of mass]). The Oshiki Kyosho theory became the standard of the later established, weights and measures system. Successive dynasties in China laid down the units of weights and measures by law. Especially, with regard to the unit of length, the length per unit became longer with the times until the length was fixed at about 3 centimeters for 1 sun in and after Tang Dynasty. Besides China, the whole region in the East Asia (including Japan, Korea) that was influenced by China adopted these units along with Chinese culture. Although the units were changed independently by each country, the value of each unit has changed little from that fixed in the period of Tang Dynasty, with remaining the original value.

All countries that once used the Shakkanho have changed their unit systems to the International Unit System, with the result that not a single country officially uses units of Shakkanho at present. However, private sectors in China and South Korea still use the units of Shakkanho. Japan now uses the units of the International Unit System, but it generally uses the shaku as a basic scale for designing Japanese-style houses, regarding the shaku as a scale appropriate to Japanese living environment. However, blueprints are drawn up by using the metric system.

Exceptionally, a unit 'monme' (or momme) is globally used as an international unit for dealing with pearl because of the confusions resulted from inconsistent unit systems: The diameter of a pearl was written in centimeter, the length of necklaces and others was in inch, the mass was in gram. In the fields of architecture and real estate, 'tsubo' (1 tsubo corresponds to a space of two Japanese tatami mats) is informally but regularly used as a unit for showing the size of land and floor. However, the unit 'tsubo' is prohibited from using directly in real estate transaction.
In showing the price of 1 tsubo when a house is constructed, for example, it is written as 'some thousand yen per 3.3 square meters' instead of 'some thousand yen per 1 tsubo.'

Units

Length and distance
The basic unit of length and distance ('measures' of the weights and measures system) is shaku. It is believed that other units came into existence separately from the shaku. However, they were connected with the shaku later, defined as unit equivalent to integral multiples of shaku or integral divisions of shaku.

The unit length of the shaku varies according to the periods and the regions. Moreover, even in the same period, several types of shaku with different unit length were simultaneously used for different purposes. Two kinds of units with the shaku remain in Japan of today, one is kane-jaku (generally called 'shaku' simply) and the other is kujira-jaku whose unit length of the shaku is 1.25 times longer than that of the kane-jaku. Refer to the article of shaku for details.

Height is measured only by the shaku. Japan Alps is about 10,000 shaku in height' is a good example. Depth is measured by hiro (6 shaku).

As for the unit of ken, The Weights and Measures Act promulgated in the Meiji period definitely provides that 1 ken is equal to 6 shaku. The ken used to only provide the module in construction works before the act was promulgated.
When 'ken' was used, it was inevitable to express the length in the units of shaku and sun, because 'ken' had no special rule but 'it is almost equal to 6 shaku.'

Besides the unit system of shaku, a unit 'mon' that was based on the diameter of the coin (Kanei Tsuho) was also used for measuring length. The diameter of 1 mon coin was fixed at almost 24 millimeter (8 bu) although there were some margins of error depending on the periods. The mon was used as a unit for measuring foot size and shoe size. 10 mon is equivalent to about 24 centimeters.

Area and acreage
Like the metric system, the unit of area is represented by the square of the unit of length as follows: 'Hosun' (the square sun), 'hoshaku' (the square shaku) and 'hojo' (the square jo).

Meanwhile, the special unit is used for acreage. The basic unit of acreage is tsubo or bu (歩). 1 tsubo or 1 bu equals the area of a square with a side 6 shaku long, namely 36 square shaku.

Based on the definition of the shaku in The Weights and Measures Act, each unit is converted to the square meter.

The units of cho, tan, se and bu are used for showing the acreage of fields, mountains, and forests, while the units of tsubo, go and shaku (勺) are used for the housing lots and the houses. Both go and shaku are originally the units for measuring volume, which are also used for measuring acreage.

1 cho, 1 tan, and 1 se extremely approximates 1 hectare, 10 are, and 1 are, respectively (practically, it is possible to use the verb "equals" instead of "approximates"). Therefore, the units of acreage were easily switched to the units of the metric system in Japan, while it was a quite difficult task for Western countries. However, it is difficult to convert the tsubo into the units of the metric system by integral multiples, so that the tsubo alone is still used. Instead of using either the go or the shaku, the acreage is shown only by the tsubo with the decimal fraction. Without using the unit of bu, the acreage of the fields and the mountains and forest are represented in are and square meter.

For fields, mountains, and forests, even though the acreage is capable of being represented only by the units of cho, tan, and se, 'bu' is usually applied to the end of those units so as to clarify the value is even.
For instance, '3 chobu' is used instead of '3 cho.'
Furthermore, 2 cho and 4 tanbu, and 6 tan and 8 sebu are good examples. For representing larger acreage than that by the cho, a unit 'hori' (a square ri; 1555.2 cho is nearly equal to 15.423 square kilometers) is used; 1 hori equals the area of a square with a side 1 ri (3,927 meters).

Volume
The basic unit of volume and capacity ('measures' of the weights and measures system) is sho (today, approximately 1.8 liters). The unit volume or capacity of the sho differs greatly according to the periods and the regions (refer to the article of sho for details), however correlation between the sho and other units has hardly changed from ancient times. It was the Edo period that the unit volume or capacity of the sho was fixed at the current value.

Based on the definition of the shaku in The Weights and Measures Act, the sho is converted to the liter.

As for measuring the volume of earth and sand, a unit ryutsubo (or tsubo in abbreviation) is used; 1 ryutsubo is equal to 6 cubic shaku. Moreover, 1 cubic shaku is also called 1 sai.

Mass
At present, the basic unit of mass ('weights' of the weights and measures system) is kan. The Weights and Measures Act promulgated in 1891 provided that the unit mass of kan is equal to 15 over 4 of the international kilogram standard (namely, 15 over 4 kilograms or 3.75 kilograms).

Ryo' was the basic unit of mass in and before the Edo period. The money-exchange business used the ryo as the basic unit of the balance weights, with using the monme as a supplementary unit. The Shirobei GOTO family was exclusively permitted to produce the balance weights throughout the Edo period while other balance weights were prohibited to be produced or used. However, the currency unit of Chogin (collective term of silver) and Mameitagin (name of an Edo-period coin) was the actual measurement value of weight. Therefore, 'monme' was used to distinguish 'ryo' for showing mass from 'ryo' as the currency unit of koban (former Japanese oval gold coin), which resulted in the wide spread of monme as a general unit for measuring mass.

Monme was used in China by its original name 'chen' (pronounced "sen" in Japanese), which was based on the mass of 1 coin (in Japan, a coin of 1 mon). The total mass of 1,000 of 1 mon coins was defined as 1 kan. As the kan was also used as the currency unit (1 kan equals 1,000 mon; in the Edo period, 1 kan equaled 960 mon under the Shohaku method [currency exchange rate in Edo period]; in the Meiji period, 1 kan equaled 10 sen), the kan for representing mass was called kanme, while that for representing the currency was called kanmon, to make clear the difference between them.

The kan was converted to the kilogram on the basis of The Weights and Measures Act. The conversion ratio in the Edo period was a little smaller than that defined in The Weights and Measures Act.

The units of mass in the weights and measures system were originally based on the mass of millet.

There is a description in "Book of Han, Treatise on rhythm and the calendar" as follows:
'Shu, ryo, kin (斤), kin (鈞) and seki are units.'
'By these units, the mass of matter is represented.'
'These units are based on the volume of Kosho.'
'The volume of Kosho is defined as 1 yaku, and the mass of 1200 grains of black mille, which can fill up the inside of the Kosho, is defined as 12 shu.'
'The mass of black mille filled 2 Kosho is 1 ryo.'
'Namely, 24 shu equals 1 ryo.'
'16 ryo equals one-kin (斤).'
'30 kin (斤) equals 1 kin (鈞).'
'4 kin (鈞) equals 1 seki.'
In short, the mass of 1200 grains of black mille is defined as 12 shu (later, the simplified spelling '朱' replaced '銖'), and two masses are defined as 1 ryo.
The Chinese character 'ryo' has a meaning of 'two.'
Therefore, 24 shu equals 1 ryo. Furthermore, 16 ryo is defined as 1 kin (斤), 30 kin (斤) is defined as 1 kin (鈞), and 4 kin (鈞) is defined as 1 seki.

There is a description in "Book of Han, Treatise on rhythm and the calendar" that the mass of Chia-ling standard measure was fixed at '4 kin' (鈞) in the weights and measures system in Han Dynasty. According to this description, 1 ryo is estimated to be around 3.8 sen (monme). However, in the period of Sui Dynasty, a new unit standard large ryo was established that 1 large ryo was equivalent to about 3 times larger than 1 ryo. In the period of Tang Dynasty, the mass decreased by about 11 percents. The conversion of the units in the Chia-ling standard measure calculated by a Chinese scholar, and the units used in the Sui and the Tang Dynasties in "History of Chinese weight and measures" written by Wu Chengluo, into the gram are respectively described as follows.

The sen (monme), a unit of mass, came into existence separately from this system. Namely, Kaigen-tsuho (copper coins in Tang Dynasty) was designed on the basis of the standard that 10 coins equals 24 shu or 1 ryo. The mass of 1 coin of the Kaigen-tsuho was defined as a 10th ryo or 1 sen (monme). However, it is difficult to mint coins with uniform mass.
Therefore, the weight of 1 sen (monme) was not based on the Kaigen-tsuho

When gold coins and Chogin were first introduced before the Kamakura period, those values were evaluated by measuring the mass. Initially, gold dust with the mass of 1 ryo was defined as 1 ryo of gold, but gradually the mass and the face value became separated from each other. The value of Kyome (old Japanese unit of measure used around Kyoto) 1 ryo of gold had already declined to 4.5 monme in the Muromachi period, while Kyome to 4.4 monme and Inakame (unit of measure in old Japan) to around 4 monme in the Azuchi Momoyama period. As for Keicho koban (oval coin in Keicho era) produced in the early Edo period, the value was defined based on the mass of Kyome 1 ryo. However, Keicho koban with inferiority in the gold content and mass was frequently produced later, which resulted in the further separation of the mass and the face value.

The Unit of Amount

Originally, 'bu' (分), one of Chinese numerals showing a decimal fraction, indicates a 10th, while 'rin' indicates a 100th. However, another unit 'wari' was used to show the percentage of a 10th in Japan.
Then, in Japan, 'bu' (分) was widely used as a unit showing a 10th of 'wari,' while 'rin' as a unit showing a 100th of 'wari.'
Therefore, 'bu' (分) is generally used for showing the percentage of a 100th, and 'rin' is a 1000th in Japan.

For instance, a 100th of the shaku, a basic unit of length, is called 1 bu (分), while a 100th of the ryo, a basic unit of mass, is called 1 fun.

However, when the Chinese numerals are interpreted literally, a 10th of the sun may indicate 1 bu (分) and a 10th of the monme may indicate 1 fun.

Bu or fun
A 100th shaku (ryo)
Rin
A 1000th shaku (ryo)
Mo (毛 or go [毫])
A 10,000th shaku (ryo)
Shi
A 100,000th shaku (ryo)

Others
In the architectural fields and others, the size of sawn timber such as plywood is sometimes called by technical terms like 'saburokuban' (3 times 6 ban) and 'yonpachiban' or 'shihachiban' (4 times 8 ban), and so on. These figures show the length represented by the unit of shaku (kane-jaku). Therefore, the former usually indicates the sawn timber with the size of 3 shaku times 6 shaku (90.9 centimeters times 181.8 centimeters), while the latter indicates the sawn timber of 4 shaku times 8 shaku (121.2 centimeters times 242.4 centimeters). However, some composite panels use the same figures for products with different sizes. For example, the former indicates a composite panel with the size of 91 centimeters times 182 centimeters, and the latter indicates that of 90 centimeters times 180 centimeters.