Sleeve length is not always based on your arm length
Sleeve lengths are not always based on your arm length, because shirt sleeves do not end at the same spot on your shoulder. And each person has different shoulder widths. Also, the shape of the sleeve armhole affects the shoulder measurement which affects the sleeve measurement. Sleeve armhole shapes can be: set-in, drop shoulders, raglan, straight, and saddle.
To allow for these differences clothing designers measure 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 only is measured from the top of the shoulder to the bottom of your wrist and is often used for casual shirt sizing.
To measure your sleeve length:
2. Measure from your shoulder to just past your wrist bone to where you would like your shirt cuff to sit. Always slightly bend the elbow when measuring to allow for extra ease. If you do not do this your sleeve length will be short because the natural position of your arm is slightly bent.
3. Add the two measurements together and round up to the nearest available sleeve length.