Have the customer wear the intended base layer and stand straight with arms relaxed at their sides.
Locate the shoulder seam (where the sleeve meets the shoulder).
Measure from the shoulder seam to the desired sleeve end point.
Choose the sleeve end point:
Wrist: measure to the wrist bone.
Cuff: measure to the top edge of the cuff (or where the cuff should sit).
Thumb length: measure to the base of the thumb (if desired).
Keep the measuring tape straight and snug along the outside of the arm without compressing fabric.
Measure the arm in the natural position (not raised or stretched).
Record the measurement in inches or centimeters.
Measure both arms and use the larger value if they differ.