Here is ASOS shoes size chart. To measure your feet stand on a level floor with the back of your heels against a straight edge or wall. Measure your foot length by placing a ruler flat on the floor straight alongside the inside of your foot from your heel to your toes. If your foot is halfway between sizes, select the larger size. You may find one foot is longer than the other (this is quite normal), please use the larger shoe size in that case.
Measure your foot correctly: