Metric System Promulgation Day

What Is Metric System Promulgation Day?

Every April 11th is "Metric System Promulgation Day" in Japan. On April 11, 1921, Japan's revised Weights and Measures Law (度量衡法, Doryo Koho) was promulgated, deciding to unify Japan's units on the metric system — the meter for length and the kilogram for mass — instead of using it alongside the traditional shakkan system. This brought Japan into alignment with the international measurement system already in use across most of Europe, facilitating trade, science, and standardization across industries.

Japan's Measurement History

Before the metric system, Japan used its own traditional measurement units: the shaku (尺, approximately 30.3 cm) for length, the momme (匁) and kan (貫) for weight, and the shō (升) for volume. These units had evolved over centuries and had deep roots in Japanese architecture, textile production, and commerce — the standard spacing of tatami mats, for example, is still sometimes described in shaku. The Meiji government recognized that Japan's integration into the global economy required metric standardization, and worked progressively toward adoption from the late 19th century: Japan joined the Metre Convention in 1886.

The Long Transition

The 1921 law did not immediately erase traditional units from daily life. It took effect in 1924, but grace periods were set for the old units, and amid opposition from supporters of the traditional system the deadline for unification was postponed again and again. The Measurement Act (promulgated in 1951, in force from 1952) replaced the old law, and from 1959 the metric system became mandatory, in principle, for transactions and certification (from 1966 for land and buildings). Even so, units like the tsubo (坪, for floor area) and the gō (合, for rice and sake) are still heard informally today.

The Global Metric System

The metric system — formally the International System of Units (SI) — is used by virtually every country in the world for science, medicine, and most everyday measurement. The United States is the best-known country where non-metric units remain common in everyday life, although it too uses the metric system in science and technology. Japan's adoption in 1921 was part of a global harmonization of measurement that has enabled modern science, international trade, and technological exchange.

Fun Facts

• The "new" shaku: Japan's 1891 Weights and Measures Law defined the traditional shaku in terms of the meter (1 shaku = 10/33 of a meter ≈ 30.303 cm), a bridge between old and new systems.
• How the meter is defined: The meter was first defined in late-18th-century France as one ten-millionth of the distance from the North Pole to the equator; today it is defined as the distance light travels in a vacuum in 1/299,792,458 of a second.
• Tatami and tradition: Traditional Japanese carpentry and architecture still use the shaku for dimensions, which is why tatami mats, sliding doors (fusuma), and room sizes are described in traditional units in cultural contexts.