Measure around your arm in the area that you would like to protect from water. I.e., Above your PICC, Midline, or dressing. Refer to the sizing below to select your ideal size:  S-M: Ideal for an upper arm circumference of 8" - 12" M-L: Ideal for an upper arm circumference of 12" - 20" For customers who use the PICCPerfect PICC Line Cover, see the size guide image for your ideal size.