Depending upon the gauge (outer diameter), medical or hypodermic needles are available in many sizes, starting from 7G and going all the way to 34G. Here, the G refers to the gauge, which is 0.18 inch for a 7G needle and 0.007 inch for a 34G needle. In-between gauges, including, but not limited to, 9G, 16G, 21G, 30G, etc are also available. Basically, the higher the needle’s gauge, the smaller its diameter and therefore the thinner it is.
Apart from gauge, needle length is also a consideration in determining the size of a needle. Often, for the same gauge, multiple needle lengths are available; so many different combinations are possible. For example, a 20G needle has a diameter of 0.035 inch, but its length could be 1-inch, 1.25-inch or 1.5-inch. The same is the case for needles of gauge sizes 21, 22, 23, 24 and 25. The high-gauge, narrow-diameter 26G needle is available in both 1.5 and 0.5 inch lengths.
The gauge and length of needle chosen by the doctor or other medical professional depends on what purpose the needle is meant to satisfy. Injections into fatty tissue below the skin require a small, short needle, so a gauge size over 25 and a length of less than 1 inch can suffice. On the other hand, for injections into muscles below the fatty tissue, a thicker and longer needle is required, so 20-22G needles that are at least 1-inch long are more suitable.