Needle gauge converter
Convert needle gauge to outer diameter in millimeters, inches and French.
That's about the same as a 21 G blood-test needle. That is about 11.7 times the thickness of a human hair.
What it means
How thick is that needle?
Nominal outer diameters from the ISO 9626 table. Thin-wall and regular-wall needles of the same gauge share the outside diameter but differ inside.
Every unit
That needle in every unit
Learn
Three things worth knowing
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Higher gauge, thinner needle
18 G is about 1.3 mm, 21 G about 0.8 mm, 25 G about 0.5 mm.
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A wire-drawing table
The scale comes from the Birmingham (Stubs) iron-wire gauge of the 1800s: each number was one more pass through a drawing plate, making the wire thinner.
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Gauge is not AWG
Electrical wire gauge (AWG) and needle gauge are different tables. A 20 AWG wire is 0.81 mm; a 20 G needle is 0.91 mm.
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Popular needle size conversions
Background
About needle gauges
Needle sizes are given in gauge, and the numbers run backwards: the higher the gauge, the thinner the needle. A 16 G blood-donation needle is 1.65 mm across; a 30 G insulin needle is 0.31 mm.
The gauge is a table rather than a formula, so values between gauges are interpolated. The colour of the hub follows the gauge too: pink for 18 G, green for 21 G, blue for 23 G.