, United States (USA) (American Colonies)
Metric Conversion Act of 1975 declared the metric system the preferred method for measurements in the United States.

On December 23, 1975, U.S. President Gerald Ford signed the Metric Conversion Act into law, marking a formal effort to encourage the adoption of the metric system in the United States. The Act declared the metric system “the preferred system of weights and measures for United States trade and commerce,” signaling a push toward alignment with international standards, where the metric system was already widely used in science, industry, and global commerce.

Importantly, the law did not mandate an immediate or universal switch from the customary units long used in American daily life. Instead, it encouraged voluntary conversion, leaving inches, feet, pounds, and gallons legally permissible for general use. The Act also established the United States Metric Board to guide and coordinate the transition in government, industry, and education, providing technical advice and resources for metric adoption.

While the Metric Conversion Act represented a symbolic step toward modernization and global compatibility, the U.S. has largely continued to rely on customary units for everyday activities. Nevertheless, the Act laid the groundwork for metric use in scientific research, medicine, manufacturing, and international trade, reinforcing the dual system that persists today and highlighting the tension between tradition and modernization in American society.

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