I'm in the USA, and when I was in grade school back in the early 70s (!) they only taught us the Metric system. To bad I was forced to halfway learn that piece of crap Imperial system because almost nobody else in the country would use it. For some reason they seem to think 16 sixteens to an inch, 12 inches to a foot, 3 feet to the yard, 1760 yards in a mile is easier than a system where everything is based on 10. (I had to look up the feet/yards to mile, and so do most people, even the ones that don't know metrics.)
If you ask me, 10 millimeters to a centimeter, 10 centimeters to a decimeter, 10 decimeter to a meter, 10 meters to a decameter, 10 decameters to a hectometer, and 10 hectometers to a kilometer, and so on is just bloody easy.
If you want to convert millimeters to kilometers, it's dead simple as it's just operations of 10, which you might be more familiar with as moving the decimal point depending on your math classes. And by the way, that is 1,000,000 millimeters is one kilometer, no calculator needed for such a simple conversion.
Now for your next trick, try converting 16ths of an inch to a mile. I'm not sadistic, you can use a calculator, and good luck. :P