Birmingham gauge

The Birmingham gauge is a wire gauge system, and is also used to specify thickness or diameter of hypodermic needles and tube products. 
Terminology.
Birmingham gauge is also known as the Stubs Iron Wire Gauge or Birmingham Wire Gauge. It is not the same as, though similar to, the Stubs Steel Wire Gauge.
Birmingham gauge is often simply termed Gauge, with the abbreviation G. However, this should not be confused with French gauge.
System.
The gauge starts at the lowest gauge number of 5Ø or 00000, corresponding to the largest size of , and runs to the highest gauge number of 36, corresponding to the smallest size of . Size steps between gauges range from between high gauge numbers to between the two lowest gauge numbers and do not correspond to a particular mathematical pattern, although for the most part the steps get smaller with increasing gauge number. Concerning wire and fine tubing, the gauge number is used to specify the outside diameter of the product, whereas for larger mechanical tubing the gauge number specifies the wall thickness independent of the overall size of the tube.
In medicine, the Birmingham gauge specifies the outside diameter of hypodermic needles, catheters, cannulae and suture wires. It was originally developed in early 19th-century England for use in wire manufacture, and it began appearing in a medical setting in the early 20th century.
Another common needle gauge system is the French catheter scale.
"Needle wire gauge" was derived from the Stubs Iron Wire Gauge.
Sizes of hypodermic needles.
Hypodermic needles are available in a wide variety of outer diameters described by gauge numbers. Smaller gauge numbers indicate larger outer diameters. Inner diameter depends on both gauge and wall thickness. The following chart shows nominal inner diameter and wall thickness for regular-wall needles. Thin-wall needles (not shown) have identical outer diameters but larger inner diameters for a given gauge.
Rapid blood transfusion through 23G or smaller needles can cause hemolysis.
Sizes of catheters.
This includes peripheral venous catheters and central venous catheters. The gauge compared to outer diameter is the same as for needles, but the color coding is different.