Nike Knee and Elbow pads size guide

Knee pads:

Measure the circumference of the knee at the strongest point with the knee slightly bent.

Size Circumference (inches)
S 14-15
M 15-17
L 17-20
XL 20-22

Elbow pads:

Measure the circumference of the elbow at its strongest.

Size Circumference (inches)
S/M 9-11
L/XL 11-14

Ankles:

Measure the circumference of the ankle.

Size Circumference (inches)
S 7
M 8-9
L 9-10
XL 10-12