Inch (unit of length)

Characteristics and conversions

An inch is used to measure short distances and small objects. By international agreement an inch equals exactly 2.54 centimetres, a relationship widely used in conversion tables and engineering drawings. There are 12 inches in a foot and 36 inches in a yard. In many technical contexts the symbol " can be seen after a numeric value; some styles prefer the two-letter abbreviation in.

History and etymology

The name comes from Middle English unche and Old English ynce, ultimately from the Latin uncia, meaning "a twelfth part". Historically the length called an inch varied by place and period; it was sometimes defined by natural references such as the width of a thumb or barleycorns. These ad hoc standards were gradually replaced by more uniform legal and scientific definitions.

Standardization and modern definitions

In the 20th century, international agreements established a fixed conversion to the metric system. The modern, exact definition—1 inch = 2.54 centimetres—is used for most civilian and commercial purposes. A related technical legacy is the separate surveying definition that existed in the United States; this produced a very slight difference in large-scale mapping but has been largely reconciled in modern practice.

Uses and examples

• Construction and carpentry: common for plans, timber sizes and fittings.
• Manufacturing and machining: small tolerances and part dimensions are often specified in inches or fractions of an inch.
• Consumer goods: screen diagonals (televisions, laptops), clothing measurements (waist sizes), and pipe diameters are frequently expressed in inches.
• Everyday reference: rulers and tape measures in many countries still show inches alongside metric marks.

Notable facts and distinctions

Although the metric system is standard in science and most of the world, the inch remains entrenched in particular industries and regions. Historically the idea of dividing larger units into twelfths links the inch to older counting systems. For readers seeking further technical or historical detail see resources on the metric conversion, historical measures in English-speaking lands (etymology), and standards development documents (standards).