Japanese units of measurement

The shakkanhō (尺貫法) is the traditional measurement system of the Japanese archipelago, largely adapted from the ancient Chinese system that spread across the Sinosphere. It has remained mostly unchanged since Japan adopted the Tang dynasty’s measures in 701. After the Meiji Restoration in 1868, Imperial Japan embraced the metric system and redefined its traditional units using a prototype meter and kilogram. The current values of most Korean and Taiwanese units also stem from these definitions.

In the early 20th century, Japan legally permitted three systems: the traditional shakkanhō, the metric system, and the Imperial system. Today, commerce is restricted to metric units, but the old measures persist in certain areas. Carpenters and farmers still use tools like chisels, spatulas, saws, and hammers marked in sun and bu sizes. Floor space is commonly expressed in tatami mats, and land is sold by the tsubo. Sake is sold in multiples of 1 gō, with common bottle sizes of 4 gō (720 mL) or 10 gō (1.8 L, called isshōbin).

Japan signed the Treaty of the Metre in 1885, which took effect in 1886, and received its prototype meter and kilogram in 1890. A weights and measures law in 1891 codified the Japanese system, making the shaku and kan the fundamental units and deriving others from them. The law set equivalent values between traditional and metric units but kept the traditional ones as the formal standard, with metric values as secondary.

In 1909, English units were also legalized in the Empire. After World War I, the Ministry of Agriculture and Commerce formed a committee to investigate which of the three systems should be adopted. Following its advice, the Imperial Diet made the metric system the legal standard as of July 1, 1924, allowing the other systems only as a transitional measure. The government and leading industries were to convert within a decade, with others following in the next decade. Public schools began teaching the metric system, and government agencies and the Japanese Weights and Measures Association promoted conversion gradually. However, opposition grew vocal in the early 1930s, with nationalists arguing the “foreign” system harmed Japanese pride, language, and culture, and restricted trade. In 1933, the government pushed the first industry conversion deadline to 1939 and the rest of the country to 1954. Emboldened nationalists then secured an Investigating Committee for Weights and Measures Systems, which in 1938 advised keeping the shaku–kan system alongside the metric one. The next year, the imperial ordinance on metric transition was revised, indefinitely exempting real estate and historical objects from conversion. The compulsory conversion deadline for all other fields was moved to December 31, 1958.

After Japan’s defeat in World War II, the American occupation brought expanded use of U.S. customary units—gasoline sold by the gallon, cloth by the yard. In June 1951, the Diet passed a Measurements Law reaffirming metrication, effective January 1, 1959. An unofficial Metric System Promotion Committee formed in August 1955, running public awareness campaigns and pushing early conversion. Its first success came in September 1956, when Tokyo department stores switched candy sales from momme to grams; NHK led media adoption. By January 1, 1959, the metric system became the sole legal system in most fields. Laws had already been rewritten with metric equivalents, but land registries took until March 31, 1966 to convert. Industry transitioned gradually at its own cost, sometimes only nominally—for example, 1/4-inch (6.35 mm) screws became “1/4 screws.” With fines for noncompliance around $140 and agencies often waiting for voluntary change, metric use by December 1959 was estimated at only 85%. Because research showed Japanese people did not intend to use metric units when alternatives existed, the sale and verification of non-metric devices (like rulers marked in shaku and sun) were criminalized after 1961.

Some traditional use continues. Some Japanese still describe their weight in kan. Homes are still reckoned in tsubo, even on the national census as late as 2005, though this practice ended in 2010. English units remain in aviation, munitions, and sports like golf and baseball.