Gauge is a unit of measurement. In body piercing, it means the diameter of the piercing hole and thickness of piercing jewelry (and thickness of the piercing needle). Each piercing is performed by a needle of a certain gauge and this needle produces a hole of the same gauge. There are many different sizes (gauges) of piercing needles that produce different sizes of piercing holes. This is very important: you need to know what gauge you are pierced with because you need to wear piercing jewelry of the exact same gauge. Gauges are given in numbers, but there is a catch: bigger sizes (larger holes, thicker jewelry) are given smaller number and smaller sizes (smaller holes, thinner jewelry) are given larger numbers. For example, gauge 18 is smaller than gauge 4. Typical piercings are performed with needles that are between 20 gauge and 10 gauge in thickness. The size (gauge) you are pierced with usually depends on the piercing type. Certain piercings are done at small gauges, such as earlobe piercings, often done in gauge 18 or gauge 16. On the other hand, genital piercings cannot be done in very small gauges so they are typically performed in gauge 12 or gauge 10. Piercing holes can be stretched to larger gauges so there are people who have their piercings stretched to gauge 8, gauge 4, gauge 0 or even bigger. Regardless of the size of the hole (big or small) you always need to wear only jewelry that is the same gauge as your piercing hole. If you are unsure about the gauge of your jewelry, there are special gauge wheels to help you. These are typically made for larger gauges (gauge 8 and up) and can be of great help to all people with stretched piercings who want to know the size of any jewelry piece.
There is some misconception about the term “gauge” and what it means for body piercings. Many people assume that “gauge” refers to the stretched piercing or jewelry made for stretched piercings. It is not uncommon to hear stretched earlobes or jewelry for stretched earlobes being called “gauges”. However, this is a mistake. The term “gauge” always refers to the size (measurement) only. Each stretched piercing has its own name and so does specialized jewelry for it. The confusion probably comes from the fact that jewelry for stretched piercings comes in large gauges. Still, the word “gauge” should be used only to refer to the measurement.