How to choose a pen needle size: 4 mm or 8 mm

• Gauge (G) is the thickness of the needle. A higher gauge is a thinner needle, so 32G is thinner than 31G.
• Length is how far the needle extends past the hub once it is screwed on. 4 mm is 5/32 of an inch. 8 mm is 5/16 of an inch, twice the length.

So the 32G 4 mm is the thinnest and shortest needle we carry, and the 31G 8 mm is slightly thicker and twice as long. Neither is "better" as a piece of hardware. They are two standard sizes, and both are sold by the same makers for the same pens.

The V2 Original Style, the V2 New Style and the 510(k) 80 unit pen take any standard screw-on pen needle. The Auto Pen takes the same standard screw-on pen needles, so our 32G 4 mm and 31G 8 mm both fit it. Our needles also fit most other brands of reusable pen.

Whichever size you pick, use a new needle every time. A used needle dulls, can clog, and lets air into the cartridge if it is left on the pen. Remove it with its outer cap after use, dispose of it in a sharps container, and put the pen cap back on. Never store the pen with a needle attached. Each new needle also needs a fresh prime; see the priming guide.

The needles are sold by the box, and the pen kits let you pick either size for the box that comes with the pen. If you are not sure which you will prefer, one box of each is the cheapest experiment. Questions go to [email protected].