First, measure the circumference of the hand, then the length of the hand from the wrist to the tip the longest finger.
Following your purchase, during your first fitting, your mitt will undoubtedly seem a little small. Note that this mitten is made entirely of leather. So it will get bigger by wearing it a few times. Plus, the sheepskin lining on the inside will flatten out. To be considered if you are in doubt about the size.
