Wrap a measuring tape around your neck (flush to the skin) at the height where the collar would be if you were wearing a shirt. Enter the exact length.
Stand up naturally with hands down at your side. Put tape around the fullest part of the chest. Tape should be snug with one finger between tape and body.
Measure from shoulder seam to other shoulder seam.
Measure from shoulder seam to the end of the shirt cuff.