Size S: neck 38-49cm, width 2.5cm
Size M: neck 41-56cm, width 3cm
Size L: neck 47-65cm, width 3.5cm
Measure the widest part of the neck, around the middle where the collar sits. Measurements are taken by hand, and a difference of 1 to 2cm is normal. Choose the size whose range your measurement sits nearest the middle of, so there are spare holes either side. Once buckled, two fingers should slide flat between the collar and the neck. New leather is firm at first and softens over the first few weeks of wear.