Place the end of the tape beside the collar at the top of the tee (Highest Point Shoulder). Pull the tape measure to the bottom of the shirt.
Measure yourself around the fullest part of your chest. Keep the tape measure horizontal.
C) Sleeve length
Place the end of the tape at the center back of the collar, then pull the tape measure along the top seam of the sleeve. When you get to the shoulder hold the tape in place at the shoulder and continue to pull down the sleeve until you reach the hem of the sleeve.