A conversion factor is a fraction equal to ' 1 '. It has the same quantity in the numerator and denominator, but they're in different units. You use it to convert a number from one unit to another unit. In order to do that, you multiply the original number by the fraction that's equal to ' 1 ', so you don't change the value of the original number, but you change its units.
Example:
How many inches are there in 7 feet ?
Use the fraction (12 inches)/(1 foot) which is equal to ' 1 ', and multiply.
(7 foot) x (12 inches/1 foot)
This multiplication produces (7 x foot x 12 x inches) / (1 foot)
When you divide top and bottom by 'foot', you have (7 x 12 inches) = 84 inches .