Definition: Needle gauge is a number for how thick a needle is. The scale runs backward: a higher gauge number means a thinner needle, so a 31G needle is thinner than a 25G one. Needle packs list gauge and length together, such as "31G × 6 mm." Gauge and length are separate choices.
A label like "31G × 6 mm" means 31 gauge thick and 6 mm long. The embecta insulin syringe range, for example, lists a 6 mm × 31G needle, with 8 mm and 12.7 mm lengths in the same line.
CDC's vaccine table gives general sizes by route:
| Route | Gauge | Length |
|---|---|---|
| Subcutaneous (under the skin), all ages | 23 to 25 | 5/8 inch (16 mm) |
| Intramuscular, adults 19 and older | 22 to 25 | Usually 1 to 1.5 inch (25 to 38 mm) |
Insulin and pen needles are thinner still, often 30 to 32 gauge. A 2013 trial compared a 32-gauge straight needle with a thinner, tapered needle with a 33-gauge tip.
Gauge and length both affect comfort, and length decides how deep the dose goes. In a 2010 trial, a 4 mm × 32G pen needle was rated less painful than 5 mm and 8 mm × 31G needles and was preferred by about two-thirds of users. A 2015 review notes that actual pain and bleeding across 21 to 31 gauge needles are mild.
If a site feels different from one shot to the next, the needle is one variable worth writing down along with the site. Dose Track logs each dose with its injection site, which makes patterns easier to spot.
Read more on Dose Track: Why injection site rotation matters · U-40 vs U-100 insulin syringes