Inch
Length · Also called: ynce, pouce, barleycorn
That is
2.54 cm
1 inch (imperial)
The other way round: 1 m is 39.3701 inches.
- Period: 1959 to today
- Region: the UK
Values
| Value | That is | Period | Used for |
|---|---|---|---|
| imperial | 2.54 cm | 1959 to today | the UK |
| pre-1959 imperialApproximate: one thirty-sixth of the standard yard bar as determined by the National Physical Laboratory in 1922. | 2.54 cm | 1824 to 1959 | the UK |
| barleycornApproximate: exactly one third of an inch, but real barleycorns vary. | 8.467 mm | c. 1300 to the 19th century | England |
What the unit was used for
The inch was defined by division rather than by an object of its own: Edward I’s Composition of Yards and Perches, dated somewhere between 1266 and 1303, set three dry round grains of barley to the inch, twelve inches to the foot and three feet to the yard.
Barleycorns were never a workable precision standard, so from 1824 the inch was simply one thirty-sixth of the imperial standard yard bar.
Its present value of exactly 25.4 mm comes from the international yard and pound agreement of 1 July 1959, although industry had already settled on 25.4 mm around 1930.
The barleycorn survives in one place a British reader still meets: UK and Irish shoe sizes step in thirds of an inch.
The inch is still lawful in the UK for road traffic signs and as a supplementary indication in trade.
In the system
1 inch = 3 barleycorns; 12 inches = 1 foot; 36 inches = 1 yard