The conversion fact that I remember is that there are 2.54 centimeters in an inch. The prefix centi means one-hundred and the prefix milli means one-thousand and 100 10 = 1000 so, there are 10 millimeters in a centimeter. Thus 2.54 centimeters is 25.4 millimeters and thus there are 25.4 millimeters in an inch.
There are 12 inches in a foot and hence 25.4 12 millimeters in a foot. How many are there is 12 feet?