Arm measurement refers to the process of measuring the length of the arm. The following are the steps to measure arm length:

1. Stand up straight with your arms relaxed and at your sides.
2. Keep your arms slightly bent, with your fingers in your pockets.
3. Measure your arm over the shoulder and down your arm. Do not measure down your back, as you will want to get the fullest arm length possible.
4. Measure between both of your middle fingers. Traditionally, arm span is measured between the middle finger of one hand to the middle finger of your other hand.

It is important to note that sleeve length is not always based on arm length, as shirt sleeves do not end at the same spot on your shoulder. Sleeve length is measured from the center back of your neck, over the top of your shoulder, and down to the point where your hand starts to widen at your wrist. The actual measurement of your arm is only measured from the top of the shoulder to the bottom of your wrist and is often used for casual shirt sizing.