We sell two pen needles, 31G x 8 mm and 32G x 5 mm, and “which one should I get?” comes up a lot. The two numbers mean different things, and one of them matters more than people expect.
The first number: gauge
Gauge is the needle’s thickness. It runs backward, so a higher number means a thinner needle. Using a standard needle gauge chart:
| Gauge | Outside diameter |
|---|---|
| 29G | 0.337 mm |
| 30G | 0.312 mm |
| 31G | 0.261 mm |
| 32G | 0.235 mm |
| 33G | 0.210 mm |
The jump from 31G to 32G looks tiny on paper, about a quarter of a millimeter down to a little under. But the inside of the needle is where it counts.
Why a small change in thickness is a big change in flow
Liquid moving through a narrow tube follows Poiseuille’s law. Harvard’s physics demonstration page puts it simply: flow is “proportional to the fourth power of the pipe’s radius.”
That fourth power is the whole story. Make the opening a little smaller and flow drops a lot. Halve the radius and you get one-sixteenth the flow at the same pressure.
That’s why our FAQ says the 32G is finer but slower. If your solution is on the thicker side, the 31G moves it noticeably faster and takes less thumb pressure.
Some needle makers work around this with thin-wall needles that keep the same outside gauge but widen the inside. BD reported that its thin-wall design reduced the force needed to push the button by about 62%.
The second number: length
The second number is simply how long the needle is. Common pen needle lengths are 4, 5, 6, 8 and 12.7 mm. BD’s lineup, for example, pairs 4 mm and 6 mm with 32G, 5 mm and 8 mm with 31G, and 12.7 mm with 29G.
Length doesn’t change flow much compared with gauge. It’s mostly about what the needle has to reach.
Will any pen needle fit?
Pen needles are made to ISO 11608-2, the standard for double-ended pen needles. “Universal” is a retail term, not a rating in the standard. The standard used to include a “Type A” label for interchangeable needles, but the 2022 edition removed it. Compatibility is now tested by how well the needle delivers and how much force it takes to remove.
Our pens accept standard screw-on pen needles from 28G to 33G.
Every needle is single-use
This is the part people skip. The standard defines pen needles as single-use, and manufacturers say so on the box. BD’s own guidance: “Pen needles should only be used once.”
Reuse isn’t just a sterility issue. Ypsomed, a needle maker, publishes magnified photos showing that after one use the tip “can be deformed” and the lubricant coating wears off. A bent or dull tip also clogs more easily, which is one of the most common reasons a pen button won’t press. A 2023 case report described a reused needle that had gone blunt, with the tip bent and its coating gone.
The other reason to take the needle off: a needle left on the pen lets air in and solution out. Every major pen manual says not to store the pen with a needle attached.
Quick picks
- Thicker solution or you want an easier press: 31G x 8 mm
- Want the finest needle: 32G x 5 mm
- Either way: new needle every time, off the pen when you’re done
Shop 31G x 8 mm or 32G x 5 mm pen needles. Done with them? Here’s how to dispose of used pen needles safely.
All PepPens products are sold for research use only.
Sources
- Hamilton Company: needle gauge chart
- Harvard Science Demonstrations: Poiseuille’s law
- ISO 11608-2:2022 Double-ended pen needles and preview on removal of “Type A”
- BD pen needle worksheet
- BD 6 mm pen needle press release
- Arthritis Foundation: BD thin-wall needle
- Ypsomed mylife: single-use pen needles
- Case report on needle reuse, PMC
