| Reference | Inches (in) | Ångströms (Å) |
|---|---|---|
| A sheet of A4 paper (long side) | 11.6929 in | 2.97 × 109 Å |
| Average adult human height | 66.9291 in | 1.7 × 1010 Å |
| A football pitch (length) | 4133.86 in | 1.05 × 1012 Å |
| A marathon | 1661220 in | 4.2195 × 1014 Å |
| Height of Mount Everest | 348386 in | 8.849 × 1013 Å |
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 ångström is a unit of length equal to one ten-billionth of a metre, or 0.1 nanometres. It takes its name from Anders Jonas Ångström, the Swedish physicist who used it in his 1868 map of the solar spectrum. The symbol is Å, a letter borrowed from the Swedish alphabet.
The ångström survives because it matches the scale of atoms. A hydrogen atom has a radius of about 0.5 Å. A carbon-carbon single bond measures roughly 1.5 Å. Expressing these figures in nanometres produces awkward decimals, so crystallographers, spectroscopists and structural biologists continue to prefer the older unit. Protein structures deposited in public databases are still described by their resolution in ångströms, and a structure resolved to better than 2 Å is considered high quality.
Wavelengths of visible light also fall in a convenient range. Red light sits near 7000 Å and violet near 4000 Å. X-ray wavelengths cluster around 1 Å, which is precisely why X-ray diffraction reveals atomic spacing: the probe and the target are the same size.
The ångström is not part of the International System of Units. The BIPM lists it among units that are accepted for use with SI but discourages new applications, preferring the nanometre or picometre. That guidance has had limited effect in the fields where the unit is entrenched. Semiconductor manufacturing offers a clear illustration. Process nodes were named in nanometres for decades, but as features shrank the industry began quoting gate oxide thicknesses in ångströms, and Intel named a generation of its technology the Angstrom era.
Reading older scientific literature requires care. Before the ångström was tied to the metre it was defined against a specific spectral line of cadmium, and figures published in the early twentieth century may differ slightly from modern values. The International Astronomical Union adopted that spectroscopic definition in 1907, and it stood until the metre itself was redefined against krypton in 1960. The discrepancy is small, but it is real, and it matters when comparing historical spectral measurements against current ones.
Converting is straightforward. One ångström equals 10-10 metres, 0.1 nanometres, or 100 picometres. Ten ångströms make a nanometre.