How do I measure a bottom bracket?
The numbers 113, 115, etc. mean the total axle length in milimeters.

On a singlespeed (fixie, internal-geared-hub), the important measure is how much the right side of the axle "shows out" (not always perfectly symmetric), and how much the crank can "sink" into the tapered square, which may vary from model to model even with the same nominal spindle length, and with how much torque you apply to the bolt when installing.

I have two single-speeds (fixie and IGH), which required some "fine tuning" to get a straight chainline:


One had a 73mm bottombracket tube on the frame, so I had to use a slightly shorter spindle, because it is installed off-center (2.5mm to the right because of extra "frame width" on the right side, the nut on the other side didn't have a lip and could be inserted fully inside the bb thread);
Other was regular 68mm, but two models with same spindle length had different offsets on the crank (because of taper, lip width of the shell, and some other obscure factors) and I had to install a cheaper model (generic instead of Shimano), it was the only one that fitted, and I'd rather have an aligned chain than a slightly better bottombracket.