To ensure a perfect knee pad fit, measure circumference of extended knee and match to the sizes listed below:JR: 9 - 11inX-Small: 10 - 12inSmall: 12 - 14inMedium: 14 - 16inLarge: 15 - 17inX-Large: 17 - 20in2-3XL: 21 - 24inPRO TIP: If your measurements are in-between the sizes shown or you’re simply in doubt, we strongly recommend sizing up to the larger size. We have found over the years that customers do better when they choose the larger size.