Many people want a smaller gauge hole in their ear than 16 gauge, so they go down to 18 gauge which is thinner than 16 gauge. 20 gauge & 22 gauge are the thickness of regular earring studs.
Cartilage hoop diameter is measured in a straight line across the center, inside the hoop. 1/4 inch diameter (6mm) is very tiny and may be OK for a tiny ear that is pierced close to the edge. 5/16 inch (8mm) is slightly larger and fits more people. 3/8 inch (10mm) is good for a piercing farther away from the edge of the ear. The only way to properly determine the size you need is to measure a piece of jewelry that fits properly and use it as a guideline! You can also wear labret studs, captive rings, circular barbells, spiral barbells, stud earrings, seamless rings, segment rings and curved barbells in your cartilage piercing.