Historically, it was defined as one minute (1/60 of a degree) of latitude along any line of longitude. Today the international nautical mile is defined as exactly 1852 metres (about 1.15 miles).

Where these systems most notably differ is in their units of volume. A US fluid ounce (fl oz), about 29.6 millilitres (ml), is slightly larger than the imperial fluid ounce (about 28.4 ml). However, as there are 16 US fl oz to a US pint and 20 imp fl oz per imperial pint, the imperial pint is about 20% larger. The same is true of quarts, gallons, etc.; six US gallons are a little less than five imperial gallons.