The United States Customary System (USCS) is the system of measurement used in everyday life, manufacturing, construction, and commerce throughout the United States. It includes familiar units such as the inch, foot, yard, mile, ounce, pound, gallon, and degree Fahrenheit. Although often referred to as the “English system,” the United States Customary System is not identical to the measurement system currently used in the United Kingdom.
The system developed from the units brought to North America by English colonists. During the nineteenth century, the United Kingdom reformed many of its traditional units through the adoption of the Imperial System in 1824. Because these changes occurred after the United States had become independent, the American system retained many earlier definitions and evolved separately. As a result, certain units, particularly those related to volume, differ between the United States Customary and Imperial systems.
Today, the United States Customary System is legally defined in terms of the International System of Units (SI). For example, the inch is defined as exactly 25.4 millimeters, and the pound is defined in terms of the kilogram. This means that although Americans commonly measure length in inches and weight in pounds, the underlying standards used by laboratories, manufacturers, and calibration facilities are ultimately traceable to SI units.
Because of its widespread use in American industry and daily life, familiarity with the United States Customary System remains important for engineers, technicians, tradespeople, and manufacturers. Understanding both US Customary and SI units allows measurements to be communicated accurately across industries, countries, and technical disciplines.