Just add inch marks to the headphones' cable, and you'll have a 31" long measuring tape that's always with you, with no extra weight or clutter.
Step 1: Tools and Materials
Soldering iron or other heat source.
Step 2: Make Inch Marks on the Headphones' Cable
Step 3: Heat the Marks
To make the marks last longer, heat them up so that the ink diffuses better into the external surface of the cable sleeve. I used 25W soldering iron, but pretty much any heat source will do.
Hold the cable right on top of the heating element of the hot iron, rotating the cable slowly. Don't let the cable touch the iron! You want the hot air ascending from the iron to make the external surface of the cable's insulation soft, but not melt it. Usually 20 to 30 seconds should be enough, but you might want to experiment a little on an extra bit of cable to determine optimum time.