The gauge refers how thick your jewelry shaft is. Jewelry shaft gets thicker the smaller the number gets. For example, an 18 gauge nose stud is thicker than a 22 gauge nose stud; An 16 gauge nose stud is thicker than an 18 gauge nose stud. You will see gauge written like this: 20G, 20g, and 20 gauges. If you don't know which gauge you wear you may be able to measure it at home or you can reach out to nose piercer to get exact size that you looking for. As long as you are wearing the correct gauge of jewelry in your piercing, just use a caliper to measure how thick (big around) your jewelry is. You cannot measure gauge accurately with a ruler. If you do not have a caliper asks your piercer for help. Here is AWG (American Wire Gauge) Measurements: 22 gauge = 0.64mm 20 gauge = 0.81mm 18 gauge = 1.02mm 16 gauge = 1.30mm The inner diameter of a hoop refers to the measurement that is taken in a straight line from inside edge to inside edge. All of the jewelry that I make is measured using the inner diameter. I use 8mm that ‘fits most’, 6mm that is ‘small’, 12mm that is ‘large’, in case of any specific measurement, please reach out to us after placing the order or leave us a note with your order.