To convert from miles to feet by hand, multiply miles by 5280
miles * 5280 = feet

To convert from feet to miles by hand, divide feet by 5280
feet / 5280 = miles

(and now the technical stuff, if you're interested)

The foot or its plural form feet, uses the symbol ft, or often simply the apostrophe '

The word mile comes from the Latin word mille, meaning 1000, since the mile was originally 1000 paces (2000 steps). Back in the year 1592 England defined the mile to be 8 furlongs, which is 80 chains, or 320 rods, or 1760 yards, or 5280 feet. This is the basic definition we still use today, the only difference being how we define the foot.

In 1959 the U.S. National Bureau of Standards and the British Weights and Measures Act of 1963 defined the foot at exactly 30.48 centimeters. Prior to that the U.S. Metric Act of 1866 defined the foot as exactly 1200/3937 meters, this is now called the survey foot.

Symbols used:
mile = mi
foot (or feet) = ft