| Reference | Inches (in) | Meters (m) |
|---|---|---|
| A sheet of A4 paper (long side) | 11.6929 in | 0.297 m |
| Average adult human height | 66.9291 in | 1.7 m |
| A football pitch (length) | 4133.86 in | 105 m |
| A marathon | 1661220 in | 42195 m |
| Height of Mount Everest | 348386 in | 8849 m |
The inch is a unit of length equal to exactly 25.4 millimetres. It is one twelfth of a foot and one thirty-sixth of a yard. The symbol is in, though a double prime is common in technical contexts and a straight double quote is often substituted informally.
The name comes from the Latin uncia, meaning a twelfth part, which also gave English the word ounce. Early definitions were anatomical or agricultural. A statute of Edward II defined the inch as three barleycorns laid end to end, and the width of a man's thumb served as a working approximation in several European languages. Dutch and Swedish still use words for inch that mean thumb.
Its modern precision dates from 1959, when the International Yard and Pound Agreement fixed the yard at exactly 0.9144 metres across the United States, the United Kingdom, Canada, Australia, New Zealand and South Africa. Before that agreement the American and British inches differed very slightly, which mattered in precision manufacturing. The United States retained a separate survey inch for land measurement until it was formally retired at the end of 2022.
The inch remains dominant in several international industries regardless of local measurement systems. Screen sizes are quoted in inches worldwide, measured diagonally. Tyre rim diameters, bicycle wheel sizes, plumbing pipe bores and rack-mounted equipment all use inch designations, and the standard 19-inch rack is a global fixture. Photographic sensor sizes use inch fractions that no longer correspond to any real dimension, a survival from television camera tube measurements.
Subdivision is traditionally binary rather than decimal. An inch is halved repeatedly into eighths, sixteenths and thirty-seconds, which suits ruler markings and fits the fractional sizing of drill bits and spanners. Decimal inches are used in machining, where thousandths of an inch are called thou or mils.
Screens and printing keep it in daily use far outside the countries that use it for anything else. Televisions, monitors and phones are sold by the diagonal in inches everywhere, so a 55-inch television has the same name in Paris as in Chicago. Typography rests on the same base: there are 72 points to the inch, a convention inherited from the printing trade and written into every computer font system, and image resolution is quoted in dots or pixels per inch. Anyone laying out a page in a country that has been metric for a century is therefore working, without particular thought, in twelfths and seventy-seconds of an inch.
One inch equals 25.4 millimetres, 2.54 centimetres, or 0.0254 metres exactly.
The metre is the base unit of length in the International System of Units, written m. Every other SI length unit is defined as a multiple or fraction of it, and units of area, volume, speed and many others are built from it in turn.
Its definition has been revised four times, each revision replacing a physical object with something more reproducible. The French Academy of Sciences proposed in 1791 that the metre should be one ten-millionth of the distance from the North Pole to the equator along the meridian through Paris. Surveying that arc took six years and produced a small error, since the Earth is not a perfect sphere. In 1889 the definition moved to a platinum-iridium bar held at Sèvres, near Paris, with copies distributed to signatory nations. In 1960 it was redefined as 1,650,763.73 wavelengths of orange-red light emitted by krypton-86.
The current definition dates from 1983 and is unusual in that it fixes a different quantity. The speed of light in vacuum is defined as exactly 299,792,458 metres per second, and the metre follows as the distance light travels in 1/299,792,458 of a second. Light speed is therefore no longer measured; it is a defined constant, and improvements in measurement now refine the metre rather than the speed.
The unit is used worldwide for building dimensions, athletics tracks, swimming pools, water depth, fabric and rope. Only a small number of countries retain imperial units for general purposes, and even there the metre dominates science and medicine.
Everyday reference points help fix the scale. A standard interior door is roughly two metres tall. A single stride for an adult is close to three quarters of a metre, which is why pacing is a workable rough measure of distance. The width of a single traffic lane is between three and three and a half metres in most countries.
One metre equals 100 centimetres, 1000 millimetres, or 0.001 kilometres. It is approximately 3.28084 feet, or 39.3701 inches.