Although the kilometer is the preferred unit of distance for the sciences and -- in most countries -- for daily use, the mile remains both legal and common in the United States. Fortunately, converting from kilometers to miles is simply a matter of multiplying by a constant. Converting from miles to kilometers is just as simple but uses a different constant.
To convert kilometers to miles, multiply the kilometer figure by 0.6213. For example, 500 km * .6213 equals 310.65 miles. You can convert miles to kilometers by multiplying by the factor 1.6093. For example, 150 miles * 1.6093 equals 241.4 km. If your calculation requires greater precision, use the multipliers 0.621372 for kilometers to miles and 1.6093419 for miles to kilometers.