A 20-gauge hypodermic needle has an outer diameter of 0.036 inches and an inner diameter of 0.024 inches. These are nominal measurements. The actual size of a 20-gauge needle may range from 0.0355 to 0.0360 inches, and the actual inner diameter may range from 0.0230 to 0.0245 inches.
The wall of a 20-gauge needle is generally 0.006 inches thick. In medicine, the gauge of a needle normally refers to its outer diameter, or the size of the puncture hole it makes. The gauge of a needle is inversely proportional to its size. In other words, a 16-gauge needle is thicker than a 20-gauge needle, and a 24-gauge needle is thinner than a 20-gauge needle. The thickness of a needle has little relationship to its length.