2,000 English idioms, phrases and expressions that we use daily, with their meanings and origins explained...
1 An abbreviation might be formed using the first and last letter of the complete word.  2 Yard, for example, is abbreviated yd., and quart is abbreviated qt.. 3  Some measurement abbreviations are formed by using the first letter or two (or sometimes three) of the word. 4  Some examples of this are mi. ( 5 mile), in.
In the United Kingdom it is easy: 1 mi = 5280 ft (by definition, so exact). In the United States, there are two kinds of miles and two kinds of feet that are relevant to this question. For both miles and feet, there is a pair of international units and a pair of survey units.