1) Remove your shoe and sock
2) Butt your heel up against the wall
3) Making sure your foot is flat on the floor, mark the spot where your big toe ends
4) Measure the distance from the wall to the mark in centimeters
5) This resulting distance if your boot size in Mondo Point (i.e. 24)

Boot Sole length is the length of the boot shell in millimeters. This measurement differs by company for similar sizes (i.e. a Nordica 26 does not have the same boot sole length as a Salomon 26). This measurement is important when mounting a ski and is found engraved on the side of the heel of the boot shell.

Half sizes don't really exist. A mondo point 25 and 25.5 have the exact same boot shell and liner. The difference is the thickness of the factory installed insole. If you use your own insole a 25 and a 25.5 will fit exactly the same.