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|