McDavid Hex Knee/Elbow/Shin Pads - McGrid (Pair)

Bend your elbow to approximately 45 degrees and measure directly over the joint. When ordering a pair, take measurements of both elbows and use the average of the results. 

Measure the knee to be protected (you likely figured that). Stand and bend your leg just less than 45 degrees, measure around the largest part of knee, directly over the patella (kneecap). 

Stand and measure the calf in question (you knew this). Measure at the fullest part of the calf. When ordering a pair, take measurements of both calves and use the average of the results.