Written and maintained by CASRAI Editorial Board
Last updated
General product information, not medical advice. Which needle suits you is a decision for your prescriber, diabetes educator or pharmacist, and the label on your own box governs.
Insulin pen needles are described by two numbers: gauge (thickness) and length in millimetres. A higher gauge number means a thinner needle. The chart below lists the gauge and length combinations that manufacturers list for pen needles in the US FDA device database, so you can match a number on a box to a size, or read an inch fraction as millimetres.
Pen needle size chart (gauge x length)
| Gauge | Length | Inch fraction seen in listings |
|---|---|---|
| 34G | 4 mm | 5/32 in |
| 33G | 4 mm | 5/32 in |
| 32G | 4 mm | 5/32 in |
| 32G | 6 mm | not stated in listings |
| 31G | 5 mm | 3/16 in |
| 31G | 6 mm | not stated in listings |
| 31G | 8 mm | 5/16 in |
| 30G | 5 mm | 3/16 in |
| 30G | 8 mm | 5/16 in |
| 29G | 12 mm (12.7 mm) | 1/2 in |
Source: records in FDA’s Global Unique Device Identification Database (GUDID) for product code FMI whose brand name includes “pen needle” (116 records, database updated 2026-10-01, checked 2026-10-30). The table shows combinations that appear in listed device descriptions. It is not a complete range, not a ranking and not a recommendation.
Check your product’s labelling. The printed gauge and length on the carton and needle are what count, not the table above.
How to read the chart
“31G 5 mm” means gauge 31, 5 mm long. The two values are independent: a gauge number says nothing about length, and length is printed separately. Most pen needle listings fall between 29G and 34G. Gauge numbers on other device types, such as syringes and catheters, use their own ranges; for the full gauge scale see the gauge and needle diameter guide.
What the FDA records say about the product
Pen needles in the database are listed under product code FMI, “Needle, Hypodermic, Single Lumen”, regulation number 880.5570, device class 2. The records are marked single-use. Pen needles are made for pen injectors, which are covered by the ISO 11608 family of standards; see ISO 11608 explained.
Gauge, millimetre and inch equivalents
- 5/32 in is about 4 mm, and the listings pair them (for example a 33G needle described as 5/32 in, 4 mm).
- 3/16 in is about 5 mm.
- 5/16 in is about 8 mm.
- 1/2 in is 12.7 mm, shown as 12 mm or 12.7 mm in listings.
Hub colours are not a safe shortcut. Colours vary between products, so read the printed gauge instead; see the note in the gauge guide.
Common mix-ups
- Higher number, thinner needle. 34G is thinner than 29G.
- Pen needle versus syringe needle. Syringe needles are sized separately from pen needles. See insulin syringe sizing for syringes.
- Gauge is not length. A needle can be thin and long or thin and short.
- Pen needle versus general hypodermic needle. Background on needle parts is in what is a hypodermic needle.
- Used needles. Pen needles are single-use; see how to dispose of an insulin pen.
FAQ
What do the numbers on an insulin pen needle mean?
The first number with a G is the gauge, or thickness; a higher number is thinner. The second is the length in millimetres, for example 32G 4 mm.
What is the shortest length in the chart?
The chart lists 4 mm needles in 32G, 33G and 34G as the shortest combinations in FDA’s device database listings.
Is 5/32 inch the same as 4 mm?
Approximately; manufacturers pair 5/32 in with 4 mm in their listings.
Can I tell the size from the hub colour?
No. Read the printed gauge and length on the packaging.
Sources
- US FDA, Global Unique Device Identification Database (GUDID), product code FMI, via openFDA: https://api.fda.gov/device/udi.json (public viewer: https://accessgudid.nlm.nih.gov/). Checked 2026-10-30.
