Flat poly bags are a cost effective way to package your product. Each flat bag has three dimensions. The first measurement is the width across the opening of the bag. The length runs from the bags opening to the base of the bag. Gauge is the third figure in a bags dimensions. It is a measure of its thickness in mils (1 mil bags, 2 mil bags, 3 mil bags, 4 mil bags, and 6 mil bags).