An inch (plural: inches; symbol or abbreviation: in or, sometimes, ″ – a double prime) is the name of a unit of length in a number of different systems, including Imperial units, and United States customary units. There are 36 inches in a yard and 12 inches in a foot. A corresponding unit of area is the square inch and a corresponding unit of volume is the cubic inch. The inch is usually the universal unit of measurement in the United States, and is widely used in the United Kingdom, and Canada, despite the introduction of metric to the latter two in the 1960s and 1970s, respectively. The inch is still commonly used informally, although somewhat less, in other Commonwealth nations such as Australia; an example being the long standing tradition of measuring the height of newborn children in inches rather than centimetres.

International inch

Effective July 1, 1959, the United States and countries of the British Commonwealth defined the length of the international yard to be 0.9144 meter. Consequently, the international inch is defined to be equal to 25.4 millimeters.

The international standard symbol for inch is in (see ISO 31-1, Annex A). In some cases, the inch is denoted by a double prime, which is often approximated by double quotes, and the foot by a prime, which is often approximated by an apostrophe. The two parts are sometimes separated by a dashFact: date=July 2008 (for example, 6 feet 2 inches is denoted by 6′-2″). In most languages the word "Thumb" means an Inch.

Equivalence to other units of length

• 1,000 thou (1 thou is 0.001 inches.)
• 1,000 mil (1 mil = 1 thou = 0.001 inches)
• 1,000,000 microinches (1 μin is one millionth of an inch.)
• about 0.08333 feet (1 foot is equal to 12 inches.)
• about 0.02778 yards (1 yard is equal to 36 inches.)
• 2.54 centimeters (1 centimeter is equal to about 0.3937 international inches.)

Historical origin

The origin of the inch is disputed. Historically, in different parts of the world (even different cities within the same country) and at different points in time, the inch has referred to similar but different standard lengths.

The English word inch comes from Latin uncia meaning "one twelfth part" (in this case, one twelfth of a foot); the word ounce (one twelfth of a troy pound) has the same origin.

In some other languages, the word for "inch" is similar to or the same as the word for "thumb"; for example, inch/thumb; inch/thumb; inch, pulgar thumb; inch, polegar thumb; inch, tumme thumb; inch/thumb; inch, anguli finger; inch/thumb.