| Micrometers (μm) | Miles (mi) |
|---|---|
| 1 Micrometer | 6.21371192237 × 10-10 mi |
| 2 Micrometers | 0.00000000124274238447 mi |
| 3 Micrometers | 0.00000000186411357671 mi |
| 4 Micrometers | 0.00000000248548476895 mi |
| 5 Micrometers | 0.00000000310685596119 mi |
| 10 Micrometers | 0.00000000621371192237 mi |
| 20 Micrometers | 0.0000000124274238447 mi |
| 25 Micrometers | 0.0000000155342798059 mi |
| 50 Micrometers | 0.0000000310685596119 mi |
| 100 Micrometers | 0.0000000621371192237 mi |
| Reference | Micrometers (μm) | Miles (mi) |
|---|---|---|
| A sheet of A4 paper (long side) | 297000 μm | 0.000184547 mi |
| Average adult human height | 1700000 μm | 0.00105633 mi |
| A football pitch (length) | 105000000 μm | 0.065244 mi |
| A marathon | 4.2195 × 1010 μm | 26.2188 mi |
| Height of Mount Everest | 8.849 × 109 μm | 5.49851 mi |
The micrometre is one millionth of a metre, written µm using the Greek letter mu. It is also widely called the micron, a name the General Conference on Weights and Measures formally abolished in 1967 but which remains in everyday use across manufacturing and materials science.
The unit covers the scale of individual cells and fine particles. A human red blood cell is about 7 µm across. Bacteria typically measure 1 to 5 µm. Human hair ranges from roughly 17 to 180 µm in diameter, which is why hair is visible while cells are not. The limit of unaided human vision falls near 50 µm.
Air quality regulation relies on the micrometre. PM10 and PM2.5 refer to particulate matter smaller than 10 µm and 2.5 µm respectively. The distinction matters medically: larger particles are filtered by the nose and throat, while PM2.5 penetrates deep into the lungs and can enter the bloodstream.
Precision engineering works in the same range. Machining tolerances are commonly specified in micrometres, and surface roughness is quoted in the same unit. Bearing clearances, paint film thickness and the flatness of optical components are all measured this way. The measuring instrument called a micrometer, or micrometer screw gauge, is named for the precision it offers rather than for a fixed relationship to the unit.
Wavelengths in the infrared are usually given in micrometres rather than nanometres. Thermal imaging cameras typically operate between 8 and 14 µm, the band where objects at everyday temperatures emit most strongly.
Filtration is specified almost entirely in this unit. Water filters are rated by the smallest particle they retain, commonly between 0.2 and 50 µm, and a 0.2 µm filter is fine enough to remove most bacteria. Surgical masks and respirators are tested against particles in the same range. The N95 designation refers to a filter that captures at least 95 per cent of airborne particles at the hardest size to trap, around 0.3 µm.
Its older name still causes confusion. Until 1967 the unit was officially called the micron and written with a bare Greek mu, and the word remains common in industry even though the conference of that year removed it from the SI. Semiconductor manufacturing carried the name for a generation: chips were described by their process node in microns, falling from ten in the 1970s to below one by 1990, at which point the industry switched to nanometres and kept going. The modern node names no longer correspond to any physical dimension on the chip, but the earlier ones did, and they were quoted in exactly this unit.
One micrometre equals 1000 nanometres, 0.001 millimetres, or 10-6 metres.
The mile is a unit of length equal to 5280 feet, 1760 yards, or exactly 1609.344 metres. The symbol is mi. It is the principal unit of road distance in the United States and the United Kingdom.
Its name comes from the Roman mille passus, a thousand paces, where a pace counted two steps. That unit measured about 1479 metres, appreciably shorter than the modern mile. The English mile grew to its present length because it was reconciled with the furlong, an older agricultural measure. Making the mile eight furlongs required 5280 feet, and Parliament fixed that figure by statute in 1593, which is why the unit is sometimes called the statute mile to distinguish it from other historical miles.
Those alternatives were numerous. The Scots mile, the Irish mile and various German and Scandinavian miles all differed, some substantially. The Swedish mil, still used colloquially, is ten kilometres.
Road signs, speed limits and vehicle odometers use the mile in the United States, the United Kingdom, Liberia and Myanmar, and speeds are given in miles per hour. The United Kingdom's position is inconsistent by design: distances and speeds are imperial while most other official measurement is metric, an arrangement that has survived repeated proposals for change.
Athletics keeps the mile as the one non-metric distance with continuing international standing. World records are ratified over the mile alongside metric distances, largely because of the historical weight of the four-minute barrier broken by Roger Bannister in 1954. The current record stands under 3 minutes 44 seconds.
Fuel economy in the United States is expressed as miles per gallon, and aviation and shipping use the separate nautical mile, which is longer at 1852 metres and unrelated in derivation.
The map of the United States is drawn on it. The Public Land Survey, begun in 1785, divided the western territories into townships six miles square, each containing thirty-six sections of one square mile, and each section of 640 acres subdividing neatly into quarters and quarter-quarters. Roads followed the section lines, which is why so much of the American Midwest is a grid of straight routes one mile apart, and why farms there are described in quarter sections rather than in acres. Anyone flying over Iowa or Kansas is looking at the mile drawn across an entire landscape, two centuries after the surveyors laid it out.
One mile equals 1760 yards, 5280 feet, 8 furlongs, or 1609.344 metres. A five-kilometre race is about 3.107 miles.