Nautical Mile
The Nautical Mile (NM) is the unit of distance used in marine and air navigation. It is defined as exactly 1,852 metres, chosen because it is very close to one minute of arc of latitude — so distances on a chart can be measured directly against the latitude scale at the side.
Speed at sea is measured in knots, one knot being one nautical mile per hour. The nautical mile and the knot underpin passage planning, voyage estimation and the speed-and-consumption figures used to run a ship commercially.
The nautical mile links distance directly to latitude on the chart, which is why navigation, passage planning and voyage estimating are all built on it. Getting distances and speeds right in nautical miles and knots is the basis of every ETA and fuel calculation.
How long is a nautical mile compared to a statute mile?
A nautical mile is 1,852 metres, about 1.15 statute (land) miles. It is used at sea because it corresponds to one minute of latitude on the chart.