How to Measure a Shirt: Chest Width
Spread the shirt out on its back. Be careful to fully spread out any wrinkles or folds that might hide underneath. If the shirt has rear pleats, this will be especially important. Put one end of the measuring tape at the bottom of the armpit of the shirt and measure straight across to the bottom of the other armpit.
While we do our best to sew each garment precisely to its specification every time, hand sewing and different fabric types naturally result in small variations. We consider a tolerance of +0.25” to -0.25” acceptable for the Chest Width dimension.