Each gauge has a set caliber. By far the most common gauges are 12 (0.729 in, 18.5mm diameter) and 20 (0.614 in, 15.6mm), this includes other more or less common gauges, such as the 10, 16, 24, 28, 32, and 67 (. 410 bore) gauge. Different gauges have different typical applications.

Gauge is determined by the number of lead balls of size equal to the approximate diameter of the bore that it takes to weigh one pound. For example, it would take 12 lead balls with the same diameter as a 12-gauge shotgun bore to weigh one pound.

A shotgun gauge refers to the diameter of the shotgun's bore, which determines the size of the ammunition. Common shotgun gauge sizes include the 10 gauge, 12 gauge, 16 gauge, 20 gauge, 28 gauge, and . 410 bore. The gauge number is inversely related to ammo size.

The bore diameter for a 10ga shotgun is 0.775". It's rare to find a length for 10 gauge shells chambered in anything other than 3 1/2," but they do exist in 3". A light waterfowl 10ga load is 1 3/8oz. In contrast, some turkey loads are as much as two ounces.