One nautical mile is exactly 1,852 metres, or 1.852 kilometres. Unlike almost every other distance unit, it was not derived from a body part or a farming practice — it was derived from the size of the Earth, and that is what makes it useful.
About the units
A nautical mile was originally defined as one minute of arc of latitude along a meridian: divide the Earth's circumference into 360 degrees, each degree into 60 minutes, and one of those minutes is a nautical mile. The elegance of this is that a navigator can read distance directly off the latitude scale on the side of a chart, with no conversion at all. Because the Earth is not a perfect sphere, the actual length of a minute of latitude varies from about 1,843 m at the equator to about 1,862 m at the poles, so an international standard value of exactly 1,852 m was adopted in 1929 and is now universal. The related unit, the knot, is one nautical mile per hour.
The exact factor
The relationship 1 nmi = 1.852 km is exact by definition. Going the other way, 1 km = 0.5399568 nmi, roughly 0.54. A practical consequence worth remembering: one degree of latitude is 60 nautical miles, or about 111 kilometres, anywhere on Earth. Longitude does not work the same way, since meridians converge — a degree of longitude is 60 nautical miles at the equator and zero at the pole, shrinking by the cosine of the latitude.
Where you meet this conversion
Marine and air navigation use nautical miles universally, including in countries that are otherwise entirely metric, because the link to latitude is too useful to give up. Aviation charts give distances in nautical miles and altitudes in feet, so both imperial-looking units coexist with metric weather data. Territorial waters are defined in nautical miles: 12 for territorial sea, 200 for the exclusive economic zone, which is 22.2 km and 370.4 km respectively. Search and rescue, fishing limits and shipping lanes all follow the same convention.