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, formally adopting the metric system — using the meter, kilogram, and second as base units — as Japan's official standard for measurement. 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.

The Long Transition

The formal adoption of the metric system in 1921 did not immediately erase traditional units from daily life. Traditional measurements persisted in construction (especially traditional architecture), land measurement, and some retail contexts for decades. Japan officially completed its transition to exclusive metric use in most contexts by the 1960s and 1970s, though units like the tsubo (坪, for land area) and the go (合, for rice and sake volume) remain in use in specific cultural contexts 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. Only the United States, Liberia, and Myanmar continue to use non-metric systems as their official primary standard (though all three use the metric system for scientific and technical work). 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.
• 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.