First, You need a measuring tape to measure the contour of your knee.

Second, stand up and measure your leg at the knee height (yes, the knee joint)

Size in cm Size in inch Size
29.00cm – 34.30cm 11.41inch – 13.50inch S
34.30cm – 35.70cm 13.50inch – 14.05inch M
35.70cm – 38.50cm 14.05inch – 15.15inch L
38.50cm – 41.5cm 15.15inch – 16.33inch XL