To find your size, place a tape measure on a flat surface up against a wall. Stand on the tape measure with your heel against the wall. Look down and take the measurement from your longest toe. Then use the MM scale on the size chart to find your size.
The mm sizes on the size chart are the exact length of the last that we use to build the shoes. If you find that your mm size is exactly the same as a value on the size chart, you may want to select one size bigger to give your toes some room. Eg. You measure your foot and it is 268mm is size which is a size 42.5. Unless you like your shoes very tight, you may wish to wear a size 43.