Here’s how to measure a T-shirt:
Length：Measure from the top of the shoulder down to the bottom hem.

Chest/Bust：Lay the T-shirt flat, measure from one armpit seam to the other, and multiply by 2.

Shoulder：Measure from one shoulder seam to the other shoulder seam.

Sleeve：Measure along the sleeve from the shoulder seam to the edge of the cuff.

Keep the T-shirt flat and use a flexible measuring tape for accuracy! 

Sizes can vary from 2-3 centimeters because they are measured by hand.