US length units look arbitrary (12 inches, 3 feet, 5,280 feet), but each number has a history. Most of them began with the human body or with farming, and were tidied up by law over several centuries.
The inch: a thumb and three barleycorns
The word inch comes from the Latin uncia, “a twelfth part”: the Romans divided their foot into twelve. Many languages use the same word for inch and thumb, a reminder of the inch’s origin as a thumb’s width. Medieval English law, traditionally dated to 1324, defined the inch as three grains of barley laid end to end, which is why the barleycorn still survives in UK and US shoe sizes.
The foot: twelve inches, give or take
Almost every ancient culture had a foot. The Roman pes was about 29.6 cm, a little shorter than today’s. Medieval Europe had dozens of different feet; the English foot of twelve inches won out in Britain and its colonies. Twelve is useful because it divides evenly by 2, 3, 4 and 6, which helps carpenters split a length into halves, thirds and quarters.
The yard and the mile
The yard, three feet, was the standard measure for cloth. A popular story says it was the distance from King Henry I’s nose to his outstretched thumb; it is probably a legend, but a yard is indeed about an adult’s nose-to-fingertip reach.
The mile comes from the Roman mille passus, “a thousand paces”, where a pace was two steps, about five Roman feet. The Roman mile was roughly 1,480 m. In 1593 an English statute fixed the mile at 8 furlongs, and since a furlong (the length of a furrow in a ploughed field) was 660 feet, the statute mile became 5,280 feet, an awkward number that is the price of agreeing with the farmers.
1959: the inch becomes exactly 2.54 cm
By the 20th century the US and UK inch differed by a few millionths, enough to cause trouble in precision engineering. In 1959 the US, UK, Canada, Australia, New Zealand and South Africa agreed that one yard is exactly 0.9144 m. It follows that one foot is exactly 0.3048 m and one inch exactly 2.54 cm, the factors behind every answer in the length converter and the feet-and-inches view of the height converter.
One exception lingered: US land surveys kept the older “survey foot” (1200/3937 m), about two parts per million longer. Over a mile it adds up to about 3 mm, but over a state-wide coordinate system it mattered. NIST retired the survey foot at the end of 2022, so there is now one foot.
The nautical mile is different
Sailors and pilots use the nautical mile, originally one minute of latitude and now exactly 1,852 m, about 1.15 statute miles. See knots and the nautical mile.
Areas follow lengths
Because the foot is exactly 0.3048 m, a square foot is exactly 0.09290304 m². That is why the area converter can also be exact, and why one square meter holds almost eleven square feet: 3.28 × 3.28, not 3.28.
Try it
- Length & distance converter — mm to miles, drawn on a dual ruler
- Height converter — cm and feet + inches, side by side
- Area converter — square feet to acres, as a floor plan