On a t-shirt, your chest measurement will stretch from the bottom of your vertical shoulder seam, across the broadest part of your chest, and to the other side.
Stand up straight with your shoulders back and your head forward. Your measurements will come out much more precise if you are using good and relaxed posture, like the way you usually stand throughout the day.Stand with your feet flat on the ground and push your shoulders back and down. Keep your chin straight and your eyes pointed Forward. Don't lift your chin up too high, or you could be messing up your stance. The tape should be loose enough to move around. Write down the number to the closest inch on your notepad.
If you measure 41 inches, then round up to the next number 42 / L