XS ski knee pads are built for youth skiers and very small adults with a knee circumference of about 30–33 cm. If you're shopping for a young racer or a petite adult, this is likely where you'll start.
To check your size, measure around the center of your kneecap with your leg slightly bent — this mimics your skiing stance and gives the most accurate number. If you're right on the edge between XS and S, size up for comfort or down for a more secure, locked-in feel.
Fit really matters here. A pad that's too loose can slide off your knee during a fall, leaving you unprotected exactly when you need it. An XS that fits properly stays put and does its job. Always check the brand's specific size chart too, since measurements vary between manufacturers.