| Nautical miles (nmi) | Micrometers (μm) |
|---|---|
| 1 Nautical mile | 1852000000 μm |
| 2 Nautical miles | 3704000000 μm |
| 3 Nautical miles | 5556000000 μm |
| 4 Nautical miles | 7408000000 μm |
| 5 Nautical miles | 9260000000 μm |
| 10 Nautical miles | 18520000000 μm |
| 20 Nautical miles | 37040000000 μm |
| 25 Nautical miles | 46300000000 μm |
| 50 Nautical miles | 92600000000 μm |
| 100 Nautical miles | 185200000000 μm |
| Reference | Nautical miles (nmi) | Micrometers (μm) |
|---|---|---|
| A sheet of A4 paper (long side) | 0.000160367 nmi | 297000 μm |
| Average adult human height | 0.000917927 nmi | 1700000 μm |
| A football pitch (length) | 0.0566955 nmi | 105000000 μm |
| A marathon | 22.7835 nmi | 4.2195 × 1010 μm |
| Height of Mount Everest | 4.77808 nmi | 8.849 × 109 μm |
The nautical mile is a unit of length equal to exactly 1852 metres. The symbol is nmi, though NM and M also appear in maritime and aviation practice. It is longer than the statute mile by about 15 per cent.
Its definition is geographical rather than arbitrary. One nautical mile corresponds to one minute of latitude, so sixty nautical miles span one degree. This makes navigation on a chart remarkably direct: a navigator can measure a distance against the latitude scale printed on the chart's edge without any conversion, because the two are the same thing. No other length unit has this property, which is why the nautical mile has resisted metrication where almost every other traditional unit has given way.
Because the Earth is slightly flattened, a minute of latitude is not constant, varying from about 1843 metres at the equator to 1862 metres at the poles. The First International Extraordinary Hydrographic Conference in Monaco adopted the round figure of 1852 metres in 1929 to settle the discrepancy. The United States accepted this value in 1954 and the United Kingdom in 1970, both having previously used slightly different definitions.
Speed at sea and in the air is measured in knots, one knot being one nautical mile per hour. The name records the method: a log line knotted at regular intervals was paid out behind a ship and the knots counted against a sand glass. Aviation adopted both units from maritime practice, and aircraft airspeed indicators worldwide read in knots.
Maritime law is written in the same unit. Territorial waters extend twelve nautical miles from the baseline and exclusive economic zones two hundred, figures set by the United Nations Convention on the Law of the Sea.
A related unit occasionally appears in older texts. The geographical mile is one minute of arc along the equator, about 1855 metres, and differs slightly from the nautical mile because it is measured on a great circle rather than a meridian. The distinction rarely matters in practice but explains small discrepancies between historical sources.
One nautical mile equals 1852 metres, about 1.15078 statute miles, or 6076.12 feet.
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.