Lay the jacket on its back. It’s important that the jacket is buttoned up fully. Use your hands to extend and flatten out the chest and torso completely. The bottom front hem (quarters) of the jacket should fall below the back hem of the jacket, such that the jacket can lay as flat as possible along the sides. Pop the collar up such that the lapels lay over each other as flatly as possible. Position the sleeves pointing up so that you can find the absolute corner where the side of the jacket meets the armpits. Measure from this point straight across the suit jacket to the opposite edge where the armpit and torso meet.
