| Millimeters (mm) | Astronomical units (au) |
|---|---|
| 1 Millimeter | 6.68458712227 × 10-15 au |
| 2 Millimeters | 1.33691742445 × 10-14 au |
| 3 Millimeters | 2.00537613668 × 10-14 au |
| 4 Millimeters | 2.67383484891 × 10-14 au |
| 5 Millimeters | 3.34229356113 × 10-14 au |
| 10 Millimeters | 6.68458712227 × 10-14 au |
| 20 Millimeters | 1.33691742445 × 10-13 au |
| 25 Millimeters | 1.67114678057 × 10-13 au |
| 50 Millimeters | 3.34229356113 × 10-13 au |
| 100 Millimeters | 6.68458712227 × 10-13 au |
| Reference | Millimeters (mm) | Astronomical units (au) |
|---|---|---|
| A sheet of A4 paper (long side) | 297 mm | 1.98532 × 10-12 au |
| Average adult human height | 1700 mm | 1.13638 × 10-11 au |
| A football pitch (length) | 105000 mm | 7.01882 × 10-10 au |
| A marathon | 42195000 mm | 0.000000282056 au |
| Height of Mount Everest | 8849000 mm | 0.0000000591519 au |
The millimetre is one thousandth of a metre, written mm. It is the smallest division marked on most rulers and tape measures, which makes it the practical floor of everyday measurement in countries using the metric system.
Technical drawing has settled on the millimetre almost universally. Mechanical engineering drawings state dimensions in millimetres by default, usually without writing the unit at all, because the convention is understood. This avoids the decimal points that centimetres would introduce and the large numbers that micrometres would require. A component 45.5 mm long is easier to read and harder to misinterpret than one 4.55 cm or 45500 µm long.
Meteorology uses the millimetre for rainfall. A reading of 25 mm means that rain would stand 25 millimetres deep on a flat surface that did not drain, which corresponds to 25 litres per square metre. Snowfall is usually reported as depth in centimetres and separately as liquid water equivalent in millimetres.
Medicine relies on the unit for tumour dimensions, wound measurement and the size of anatomical structures on imaging. Ophthalmic prescriptions specify pupillary distance in millimetres. Blood pressure retains the older unit of millimetres of mercury, a pressure measurement whose name preserves the height of a mercury column.
Paper, sheet metal and glass are specified by thickness in millimetres. A standard sheet of office paper is about 0.1 mm thick. Domestic window glass is commonly 4 mm. Firearm and ammunition calibres are frequently given in millimetres, as in 9 mm.
Fastener sizes follow the unit closely. Metric bolts and screws are designated by their nominal diameter in millimetres, so an M8 bolt has an eight-millimetre thread. Spanner and socket sizes are marked the same way, measuring across the flats of the fastener head. This is why a metric toolkit contains an unbroken run of sizes rather than the fractional inch steps of an imperial set.
Medicine measures pressure with it as well as distance. Blood pressure is reported in millimetres of mercury, the height of a mercury column the pressure would support, so a healthy reading of 120 over 80 refers to two lengths rather than to any force directly. The convention dates from the mercury manometer and has survived every instrument that replaced it, because the numbers are familiar to every clinician alive. The same unit measures intraocular pressure in the eye, central venous pressure and the partial pressures of gases in blood, and it appears in vacuum work as the torr, which is one millimetre of mercury under another name.
One millimetre equals 1000 micrometres, 0.1 centimetres, or 0.001 metres. It is roughly 0.03937 inches, so 25.4 millimetres make exactly one inch.
The astronomical unit is a length equal to exactly 149,597,870,700 metres, or very nearly 150 million kilometres. The symbol is au. It approximates the mean distance between the Earth and the Sun and is the standard measure for distances within the solar system.
The unit was originally defined by that orbital relationship rather than by a fixed number. Earlier definitions tied it to the properties of a hypothetical body orbiting the Sun, which meant its value depended on the gravitational constant and was subject to revision as measurements improved. The International Astronomical Union ended that dependence in 2012 by fixing the astronomical unit as an exact number of metres. The change simplified calculations and removed an inconvenience: a unit whose length shifted whenever a physical constant was refined.
Determining its value was one of the great problems of observational astronomy. Transits of Venus across the Sun's disc, observed from widely separated points on Earth, allowed the distance to be triangulated. Expeditions were mounted for the transits of 1761, 1769, 1874 and 1882, and James Cook's first Pacific voyage was organised around the 1769 event. Radar ranging to Venus in the 1960s eventually settled the figure far more precisely than any optical method.
The unit makes solar system distances legible. Mercury orbits at 0.39 au, Mars at 1.52 au, Jupiter at 5.2 au and Neptune at 30.1 au. The Kuiper Belt extends to roughly 50 au. Voyager 1, the most distant human-made object, has passed 165 au. Light takes about 499 seconds to cover one astronomical unit, so the Sun is a little over eight light minutes away.
Beyond the solar system the unit becomes unwieldy, and astronomers switch to light years and parsecs. One parsec is 206,265 au.
Spacecraft navigation depends on the unit being exact rather than approximate. Trajectories to the outer planets are computed over distances of tens of astronomical units, and an error in the length of the unit itself would propagate into every position calculation. Fixing the value in 2012 removed that source of drift from the ephemerides used for mission planning.
One astronomical unit equals 149,597,870.7 kilometres, or about 92.956 million miles.