A Swedish toolmaker settled the length of the inch before any government did
By the early twentieth century, the British and American inches differed by a few millionths. In 1912, Carl Edvard Johansson, the Swede who invented precision gauge blocks, simply picked 25.4 millimetres for his inch-sized sets. Factories everywhere bought his blocks, and within decades national standards bodies caught up with the toolmaker's choice.
The inch began as a fraction. Its name comes from the Latin uncia, meaning a twelfth, which also gave English the word ounce. Across much of Europe, though, the word for inch is simply thumb, or close to it: Swedish tum and tumme, Spanish pulgada and pulgar. A man's thumb is roughly an inch wide, and King David I of Scotland reportedly defined the Scottish inch around 1150 as the average thumb width of a small, a medium and a large man.
England preferred grain. From 1066 an inch was three barleycorns, and a statute of Edward II in 1324 specified three dry, round grains laid end to end. The earliest known English mention is grimmer: the Laws of Æthelberht, from the early seventh century, fined offenders a shilling for each inch of wound depth. By the 1840s writers noted the obvious flaw. Barley varies, so recovering a lost inch by averaging many grains could introduce errors of up to a tenth of an inch per yard.
Physical rods replaced seeds, and the official standard yard was destroyed by fire in 1834; a replacement chosen in 1845 was copied and one was sent to America. The United States later tied its inch to the metre instead, leaving the two nations' inches very slightly apart. After Johansson's compromise, British standards adopted 25.4 millimetres in 1930 and American ones in 1933, and by 1935 industry in 16 countries used this industrial inch. Governments formalised it between 1951 and 1964, although American surveyors kept a fractionally longer inch until 2023.
The unit endures in odd corners of metric life. Screens are measured diagonally in inches worldwide, including in Japan's official standard for electronic parts, and wheel rims and pipes follow inch-based sizing. Traditional inch fractions use halves, quarters and eighths, so two and three-eighths is never written as two and six-sixteenths.
Source: Inch
