The measurement is obtained by laying the t-shirt (hoodie, shirt or other top) flat on a flat surface.
Chest Measurement half-chest and across-chest measurement
Size Guide
How To Measure Your Body: Chest
How to Measure Chest Size: 11 Steps (with Pictures) - wikiHow
Measuring: Chest Size
Body measurements A: NECK- measure at base of neck. B: CHEST
Neeva D London, Measuring Guide
Size converter - Pirates Cave
Size Guide: Helly Hansen Baselayer Top, Jacket, Midlayer & Top
Taking Measurements