Needle Gauge Chart: 18G to 32G
The bigger the gauge number, the thinner the needle: 18G is thick and used only for drawing liquid, 31G is the hair-thin needle on 0.3 mL and 0.5 mL insulin syringes, and 32G is the usual pen needle. Length is the second number, written after the gauge, as in 31G × 8 mm (5/16 in).
Last edited · written by PepPrep · not medical advice
Gauge runs backwards: the bigger the number, the thinner the needle. Once that is in your head, the rest of the chart is easy.
| Gauge | Outer width | Feels like | Used for |
|---|---|---|---|
| 18G | 1.27 mm | Thick — never for skin | Drawing water into a syringe; blunt-tip versions are the safe choice |
| 21G | 0.82 mm | Thick | Drawing up; some intramuscular use by clinicians |
| 25G | 0.51 mm | Medium | Intramuscular injections (see the sub-Q vs IM guide) |
| 27G | 0.41 mm | Fine | Luer-lock needles for injecting from a 1 mL or 3 mL syringe |
| 29G | 0.34 mm | Very fine | Older and larger-barrel insulin syringes |
| 30G | 0.31 mm | Very fine | 1 mL insulin syringes |
| 31G | 0.26 mm | Hair-thin | 0.3 mL and 0.5 mL insulin syringes — the common choice |
| 32G | 0.23 mm | Hair-thin | Pen needles (4 mm is the popular length) |
Thinner is not always better
A thinner needle stings less, but liquid moves through it more slowly and it bends more easily. Drawing 2 mL of water through a 31G needle is slow enough to make people impatient and pull the needle out of the vial; that is what the 18G blunt fill needle is for. Draw with the thick one, swap to the thin one — or just use an insulin syringe, which does not swap at all.
Length is the other number
Written after the gauge: 31G × 8 mm, 27G × 13 mm. For under-the-skin injections the short lengths (4–8 mm) are the norm, and 12.7 mm (1/2 in) is an older under-the-skin length. Needles for muscle are longer, typically 25 mm (1 in). Length has nothing to do with how much you inject. The sub-Q vs IM guide explains why the two routes want different lengths, and the syringes guide puts gauge, units and barrel size together.
Supplies mentioned: Syringes & Needles
Equipment only: nothing on this page is advice about what to inject, how much, or whether a product is fit to use. That comes from the product's labelling and whoever supplied it.