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Metrication, or the process of adopting the International System of Units (SI) as the main system of weights and measures, has had a complex history in the United States. While most countries around the world have adopted the metric system, the U.S. still operates mainly under the U.S. Customary System. The push for metrication in the U.S. began as early as the 19th century, yet despite several legislative efforts, such as the Metric Conversion Act of 1975, adoption has been slow and incomplete. These efforts aimed to incrementally integrate the metric system into American society; however, the lack of a definitive legal mandate for its exclusive use has allowed the customary units to persist. One of the most visible efforts is the requirement for dual labeling of goods, showing both customary and metric units, but even this has not led to a widespread shift in everyday use by the American public.

The U.S. Customary System is the predominant system of weights and measures used in the United States. It's a collection of unstandardized units inherited from the British imperial system, such as feet for distance, pounds for weight, and gallons for volume. Historically, these units were used in commerce and daily life, becoming deeply ingrained in American culture. Even today, these units appear in infrastructure, trade, and education, reflecting a longstanding tradition. Industry-specific custom units are also widely used; for instance, the aviation industry in the U.S. largely uses nautical miles and knots for distance and speed, respectively. The familiarity and established use of these measurements contribute to the resistance against a full-scale conversion to the metric system.

Resistance to change in adopting the metric system in the U.S. is multifaceted, ranging from cultural to practical reasons. The customary units are seen as a part of national identity, and many Americans have grown up learning and using these units exclusively, making the prospect of changing to metric seem daunting. Moreover, the cost of converting road signs, manufacturing equipment, and educational materials to metric units would be enormous. On an individual level, the learning curve associated with new measurements and the corresponding need to change habits and perceptions are significant. These factors, along with a sentiment of 'if it's not broken, don't fix it,' have contributed heavily to the resistance to full metrication in the U.S.

The historical context of the U.S. Customary System dates back to the colonial era, where British imperial units were widely used. After the American Revolution, there was a desire to unify and standardize the units of measure. However, the newly independent states had already developed a level of comfort with the existing system. Over time, these units evolved separately from the British system and became the U.S. Customary System. Though Thomas Jefferson proposed a decimal-based measurement system after independence, which would have aligned more closely with the metric system, his suggestions were not adopted. Instead, the traditional use of customary units continued, and this historical preference for the familiar has played a significant role in maintaining these units as the standard means of measurement into the modern era.