Conversion from 4 Millimeters to Nautical miles

Information about the Millimeter (mm)

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.

Information about the Nautical mile (nmi)

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.