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 English units, Imperial units, and United States customary units. Its size can vary from system to system. 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 the virtually universal unit of measurement in the United States, and is very commonly used in Canada and the United Kingdom. In the US, Canada and the UK, personal heights are expressed in feet and inches by people of all ages. In Canada, personal heights are shown in metric units on most official documents such as a person's driver's licence in Ontario and Québec. However, there are exceptions like Saskatchewan, which lists height on its drivers' licences in feet and inches.

International inch

In 1958 the United States and countries of the Commonwealth of Nations defined the length of the international yard to be 0.9144 metres. Consequently, the international inch is defined to be equal to 25.4 millimetres.

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 then separated by a dashFact: date=July 2008. For example, 6 feet 2 inches is denoted by 6′-2″.

Equivalence to other units of length

• 1,000 thou (1 thou is 0.001 inches.)
• about 0.08333 feet (1 foot is equal to 12 inches.)
• about 0.02778 yards (1 yard is equal to 36 inches.)
• 2.54 centimetres (1 centimetre is equal to about 0.3937 international inches.)

Use of the inch

Even in countries where the metric system is commonplace, the inch is still sometimes used to refer to

• The size category of computer displays, image sensors, and television screens,
• Threads and diameters of pipes (see ISO 228 and Nominal Pipe Size),
• Bicycle and automobile tires and rims (see ISO 5775 and Tire code),
• Gramophone records,
• Width of computer floppy diskettes,
• Diameter of cigars,
• Clothing dimensions.
• Scale Lengths of stringed musical instruments
• Cymbal diameters
• Loudspeaker diameters
• height, with feet