Inches Conversion

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  • in
  • " (a double prime)
  • (For example, six inches can be symbolised as either 6in or 6").

Unit of:

  • Length / distance

Worldwide use:

  • The inch is both an imperial unit and part of the US system of customary units and as such the inch has widely been used in the past. The inch is still a common unit in the US, and is popular in Canada and the UK. Japanese manufacturers such as Sony and Toshiba commonly use inches to describe the size of monitors.


The inch is a unit of length used primarily in the imperial and U.S. customary measurement systems, representing 1/12 of a foot and 1/36 of a yard.


Use of the inch can be traced back as far as the 7th century. The first explicit definition we could find of its length was after 1066 when it was defined as the length of three barleycorns. This was not a satisfactory reference as barleycorn lengths vary naturally. The British Standards Institute defined the inch as 25.4mm in 1930 in the document "Metric Units in Engineering: Going SI". In March 1932 the American Standards Association were asked to rule on whether to adopt the same value (at the time the American inch was 1/.03937 mm which approximated to 25.400051 mm). Because the values were so close, and because Britain has already settled on that value, the ASA adopted this value on March 13, 1933.


The inch has been used as a unit of measurement in the United Kingdom since at least the seventh century, and in 1066 was defined as being equal to the length of three dried barleycorns placed end-to-end (a definition which survived for several centuries).

In the 12th Century the Scottish inch was defined as being equivalent to the width of an average man's thumb at the base of the nail. Similar units of measurement existed in many areas of what is now modern Europe, with the word for inch in Portuguese, French, Italian, Spanish and numerous other languages being the same or very similar to the word for thumb.

The English word inch derives from the Latin uncia, meaning one-twelfth part (an inch traditionally being 1/12 of a foot).

Even in the twentieth century various definitions of the inch were still applied around the world, although these differed by less than 0.001%. In 1930 the British Standards Institution adopted an inch of exactly 25.4mm, with the American Standards Association doing likewise in 1933, and the first country to legally adopt this definition was Canada in 1951.

In 1959 the United States and British Commonwealth countries signed a treaty agreeing to the standardised 25.4mm definition.

The English word inch is derived from the Latin word uncia, which means “one twelfth” – Because an inch was defined as being one twelfth of a foot. The word inch in other languages is often similar to the word ‘thumb’, as thumb width was a measure used to define an inch – for instance King David 1 of Scotland defined the inch as being “the breadth of the thumb of a middle sized man, measured at the root of the nail’ . In England we can find the definition of an inch as the length of 3 barleycorns in a statue of Edward 2nd in 1324.

Common references:

  • A United States quarter (25 cent) coin is just under one inch in diameter.
  • A fully-grown human eyeball is roughly one inch in diameter.
  • 1 foot, 12 inches
  • 1 metre, 39.37008 inches
  • 45 RPM vinyl singles – the popular way to buy music from the 1960s to 1990s – Normally had a diameter of 7 inches. Long players (LPs) normally had a diameter of 12 inches
  • Rulers normally come marked for 30cm and 12 inches, since 12 inches = 30.048 cm
  • A can of Coke is 4.83 inches high

Usage context:

In 1995 in the UK the inch (along with the foot, yard and mile) was officially stated as the primary units of measurement for road signs and related measurements of distance and speed. In other contexts metric measurements are now the primary system, although inches are still often used informally, particularly by people who were born and educated in pre-decimal Britain.

In the United States, surveyors use the U.S. Survey inch, defined as 1/39.37 of a metre, derived from the Mendenhall Order of 1893 that equated 1 foot with 1200/3937 meters.

Component units:

  • The inch is traditionally the smallest whole unit of length measurement in the imperial system, with measurements smaller than an inch being stated using the fractions 1/2, 1/4 , 1/8, 1/16, 1/32 and 1/64 of an inch.
  • In the UK in the early 19th century precision engineers started to use one thousandths of an inch as greater measuring accuracy became possible, and multiples of this new fraction subsequently became known as a thou.


  • 12 inches = 1 ft (foot)
  • 36 inches = 1 yd (yard)