Needle Gauge Chart: 18G to 31G, Drawn to Scale
Written and reviewed by Evan Marlow, developer of DosePlot · Updated 2026-08-20
Needle gauge measures the needle's diameter, and the scale runs backwards: a higher gauge number means a thinner needle. An 18G drawing needle is 1.20 mm across, while a 31G insulin needle is 0.25 mm — about a fifth the thickness. Thicker needles move liquid faster; thinner needles are more comfortable.
How big is each needle gauge, really?
This chart draws every common gauge at its true relative thickness, using the nominal outer diameters from ISO 9626, the standard that defines needle tubing sizes.[1]Most text tables can't show you how dramatic the difference is — seen to scale, it's obvious why a 25G injection feels nothing like an 18G would.
Which needle gauge is used for what?
In self-injection contexts, gauges cluster into three jobs: wide needles (18–21G) for pulling liquid out of vials quickly, mid-range needles (22–25G) for intramuscular injections,[2]and fine needles (25–31G) for subcutaneous injections and insulin syringes. The table below summarizes the common pattern — your prescription or provider's instructions always take precedence.
| Gauge | Outer diameter | Typical use |
|---|---|---|
| 18G | 1.20 mm | Drawing up only — never injected for these uses |
| 20G | 0.90 mm | Drawing up viscous oils quickly |
| 21G | 0.80 mm | Drawing up; blood draws in clinics |
| 22G | 0.70 mm | IM injections (CDC range starts here) |
| 23G | 0.60 mm | IM injections of oil-based compounds |
| 25G | 0.50 mm | IM (slow with oils); subcutaneous |
| 26G | 0.45 mm | Subcutaneous injections |
| 27G | 0.40 mm | Subcutaneous; some insulin syringes |
| 28G | 0.36 mm | Insulin syringes |
| 29G | 0.33 mm | Insulin syringes (very common) |
| 30G | 0.30 mm | Insulin syringes / pen needles |
| 31G | 0.25 mm | Insulin syringes / pen needles (thinnest common) |
Diameters are the designated metric size per gauge in ISO 9626:2016, Table 1; the standard specifies a min–max manufacturing range for each gauge.[1]
Why draw with a thick needle and inject with a thin one?
Two different jobs, two different tools. Drawing through a wide 18–20G needle is fast, especially with viscous oil-based compounds. Injecting with a fresh, finer needle is more comfortable — and it means the needle entering skin is factory-sharp, not one that has already been pushed through a vial's rubber stopper (a widely followed practice in community self-injection guides).
Needles are single-use, full stop: one needle, one syringe, one time — reuse carries real bloodborne-infection risk.[3] Users also widely report that reused needles feel duller and hurt more (community reports aggregated from reddit, MESO-Rx, and similar forums).
More in this guide
Part of the complete guide to syringes and needle sizes.
Sources
- ISO 9626:2016 — Stainless steel needle tubing for the manufacture of medical devices, Table 1 (designated metric outer diameters by gauge).
- CDC/ACIP — General Best Practice Guidelines for Immunization, Vaccine Administration chapter (IM needle gauge 22–25G).
- CDC — One & Only Campaign: one needle, one syringe, only one time.
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