Length Converter: cm, Inches, Meters, Miles

Screen diagonals, car rims, bike frames and plumbing diameters remain standardised in inches worldwide. These sectors adopted Anglo-American industrial standards before the metric system spread, and never switched.

No. A statute mile is 1,609 m, a nautical mile 1,852 m, about 15% longer. The nautical mile equals one minute of latitude and is used only for sea and air navigation, where speeds are given in knots.

A4 is 210 by 297 mm, US Letter is 8.5 by 11 inches, that is 215.9 by 279.4 mm: six millimetres wider, eighteen shorter. The A series follows a ratio equal to the square root of two, so folding a sheet in half gives the next size down with the same proportions, and A0 measures exactly one square metre. Letter has no such property: printing an A4 document on Letter paper scales it to about 94% and shifts every margin.

No, and the gap is a factor of 39. A mil, called a thou in Britain, is one thousandth of an inch, so 0.0254 mm: a sheet sold as 5 mil is 0.127 mm thick, not 5 mm. The micron traps fewer people, being simply the micrometre, one thousandth of a millimetre, still alive in filters and films although the CGPM dropped the name in 1967. Below it sit the nanometre and the angstrom of 0.1 nm, and note that the 3 nm of a chip has measured nothing physical for years: it is a generation name.

For the solar system, the astronomical unit, fixed since 2012 at exactly 149,597,870.7 km, roughly the Earth to Sun distance. Beyond it lies the light year, the distance light covers in a Julian year, 9.46 trillion kilometres. Astronomers themselves prefer the parsec, 3.26 light years or 30.86 trillion kilometres, because it falls straight out of a parallax measurement: one parsec is the distance at which the Earth's orbit spans one arcsecond. Proxima Centauri, the nearest star, sits at 4.25 light years, that is 1.30 parsecs.