Metrication in the United States

Metrication is the process of introducing what is now the International System of Units, also known as SI units, to replace a jurisdiction's traditional measuring units. U.S. customary units have been defined in terms of metric units since the 19th century, and, according to United States law, the SI has been the "preferred system of weights and measures for United States trade and commerce" since 1975.

The U.S. has a national policy to adopt the metric system. Under Executive Order 12770, signed in 1991 by President George H.W. Bush, all U.S. agencies were required to adopt the metric system. However, conversion was made not mandatory, so U.S. customary units remain in common use in government (for example, speed limits are still posted in miles per hour) and in many industries. There is government policy and metric (SI) program to implement and assist with metrication, but there is strong social resistance to further metrication.

In three important domains, cooking/baking, and distance and temperature measurement, customary units are generally used rather than metric.

However, the SI is used extensively in fields such as science, medicine, electronics, the military, automobile production and repair, and international affairs. The U.S. uses metric measures in photography (35 mm film, 50 mm lens), medicine (1 mL of medication), nutrition labels (grams of fat), bottles of soft drink (liters), and volume displacement in engines (liters).

The scientific and medical communities use metric units almost exclusively, as does NASA. All aircraft and air traffic control use only Celsius temperature at US airports and while in flight. Post-1994 federal law also mandates most packaged consumer goods be labeled in both customary and metric units. The U.S. has fully adopted the second, the SI unit for time.