Measure at the fullest part of the chest, under the armpits, and over the shoulder blades, keeping the tape measure firm and level.
Absolutely love the quality shirts. I had ordered a size larger since I thought they would fit tight but they are pretty true to size. I received them in the mail and reached out to customer service (Kelly I believe) and they sent me a return label since they fit too big. I then had them send me my actual size and they are perfect. Great customer service and great product!