No time for rackspace math? Use the Middle Atlantic Rack Ruler instead. It's a genuinely handy tape measure incremented in both rackspaces (red) and inches (black) down to 1/32" scale. Its Mylar-coated retracting steel tape is housed in a tough ABS case with a thumb lock and belt clip.
On the bottom of the tape they have included every formula you could ever need. It's like a cheat sheet...Ohm's Law....Cable Bend Radius....Fraction to Decimal conversions! Includes a belt clip and nylon strap to keep it close.In addition to the thumb lock there is also a quick-hold button on the bottom.