Inch (in)
The inch is a unit of length in the imperial and US customary systems, equal to exactly 25.4 millimeters or one-twelfth of a foot. It is one of the most commonly used small-scale measurement units in the United States, appearing on rulers, tape measures, and screens. From display sizes and tire widths to pipe diameters and photo prints, the inch is deeply embedded in American manufacturing and daily life.
Definition
One international inch is defined as exactly 25.4 millimeters, 2.54 centimeters, or 1/12 of a foot. Inches are commonly subdivided into halves, quarters, eighths, sixteenths, and thirty-seconds using fractional notation rather than decimals. In engineering and machining, thousandths of an inch ("thou" or "mils") are used for precision work.
History
The word "inch" derives from the Latin uncia, meaning one-twelfth, reflecting its origin as one-twelfth of a Roman foot. In medieval England, the inch was informally defined as the width of a man's thumb or the length of three barleycorns placed end to end. King Edward II formalised the barleycorn definition around 1324. Over the centuries, the inch varied slightly between countries until 1959, when the International Yard and Pound Agreement fixed it at exactly 25.4 millimeters. This definition remains in force worldwide.
Common Uses
Screen sizes for televisions, monitors, laptops, tablets, and phones are measured diagonally in inches worldwide, even in metric countries. In the United States, inches appear on rulers, lumber dimensions (a "two-by-four" is roughly 1.5 by 3.5 inches), and plumbing pipe sizes. Tire rim diameters are specified in inches globally. Paper sizes in North America use inches (Letter is 8.5 by 11 inches). In the UK, inches are used informally for small measurements alongside centimeters.
Did You Know? Facts About Inch
- The symbol for inches (″) is called a double prime and originally represented two tick marks scratched into wood by carpenters.
- TV and monitor sizes are measured diagonally - a 55-inch TV is 55 inches from one corner to the opposite corner, not across the width.
- The barleycorn, defined as one-third of an inch, is still the basis of shoe sizing in the UK and US - each shoe size is one barleycorn apart.
- A US quarter coin has a diameter of almost exactly one inch (0.955 inches).
- Despite using the metric system, Japan and South Korea advertise display sizes in inches because it has become the global industry standard.