Regular earrings are generally 20 gauge, which is 0.0320 inches or 0.812 millimeters in diameter. The needle that does the actual piercing is typically a 14 gauge. In body jewelry, the larger the gauge, the smaller the diameter. Earrings worn on the cartilage of the ear are commonly 16 gauge.
Different body piercings require jewelry of differing gauges. Lip and eyebrow rings are typically 16 gauge, belly and tongue piercings are generally 14 gauge and eyebrow rings are usually 16 gauge. Most nose rings are 18 gauge. However, there are many people who prefer smaller nose studs, so 20 and 22 gauge are also fairly popular.