Disclaimer: This article is for informational purposes only and does not constitute medical advice. Always consult a qualified healthcare professional before starting any injection therapy, changing your medication routine, or if you have any health concerns.

Needle gauge is one of those things that seems confusing until someone explains it clearly. The number on a needle refers to its thickness, but it works backwards: higher numbers mean thinner needles. A 31G needle is hair-thin and barely noticeable. An 18G needle is thick enough to draw viscous oil from a vial. Choosing the right gauge matters for comfort, speed, and making sure your medication actually reaches the right tissue layer.

This guide covers every common gauge you will encounter for self-injection, what each one is used for, and how to match needle size to your specific medication and injection method.

Various needle gauges from 18G to 31G arranged by size on a clean surface
Needle gauges from 18G (thick, for drawing) to 31G (fine, for subcutaneous injection).

1. How Needle Gauge Works

Needle gauge (abbreviated "G") measures the outer diameter of the needle. The scale is counterintuitive: the higher the gauge number, the thinner the needle. This comes from the wire gauge system, where the number originally referred to how many times metal was drawn through a die to reach a given thickness. More draws meant thinner wire.

In practical terms:

  • 18G = thick (1.27mm outer diameter). Used for drawing viscous liquids from vials.
  • 25G = medium (0.51mm). Common for intramuscular injections.
  • 29G-31G = very thin (0.33-0.25mm). Standard for subcutaneous injections and insulin delivery.

The other critical measurement is needle length, which determines how deep the needle penetrates. Subcutaneous injections need short needles (4-8mm) because the target is the fat layer just below the skin. Intramuscular injections need longer needles (25-38mm) to reach muscle tissue beneath the fat.

2. Needle Gauge Chart

This chart covers the gauges most commonly used for self-administered injections. Clinical and surgical gauges (14G-17G) are not included as they are not relevant to home use.

Gauge Outer Diameter Colour Code (ISO 6009) Common Use Pain Level
18G 1.27mm Pink Drawing thick solutions from vials; peptide reconstitution Not used for injection
20G 0.91mm Yellow Drawing medications Moderate
21G 0.82mm Green IM injection (not stocked here -- source from pharmacy) Moderate
22G 0.71mm Black IM injection (not stocked here -- source from pharmacy) Moderate
23G 0.64mm Blue IM injection (not stocked here -- source from pharmacy) Mild-moderate
25G 0.51mm Orange IM injection (not stocked here -- source from pharmacy) Mild
27G 0.41mm Grey SubQ injection, some IM (thin solutions) Minimal
29G 0.33mm Red SubQ injection, insulin, peptides Minimal
30G 0.31mm (manufacturer-specific -- check packaging) SubQ injection, insulin Very low
31G 0.25mm (manufacturer-specific -- check packaging) SubQ injection, insulin, GLP-1 medications, SubQ TRT Very low

Colour codes follow ISO 6009. Always verify the gauge printed on the packaging -- never select by colour alone.

3. Needles for Subcutaneous Injection

Subcutaneous (SubQ) injections deliver medication into the fatty tissue between the skin and the muscle. This is the injection method used for most self-administered medications, including:

You can work out the volume with our dose volume calculator.

  • GLP-1 medications (compounded semaglutide, tirzepatide)
  • Peptides (BPC-157, TB-500, and others)
  • Insulin
  • Subcutaneous TRT protocols (where prescriber has directed this route)
  • HCG and other fertility medications

Recommended gauge: 29G to 31G
Recommended length: 4mm to 8mm (6mm is the most versatile)
Recommended syringe: 1ml insulin syringe with 31G 6mm needle or 0.5ml insulin syringe with 31G 6mm needle

At this gauge and length, the needle is thin enough that most people barely feel insertion. The 6mm length is ideal for the abdomen, where the subcutaneous fat layer is typically 10-20mm deep in adults. With a 4mm or 6mm needle, 90-degree injection into a pinched skin fold is appropriate for most adults, including lean individuals (FIT/ADA guidance). 45-degree angle is recommended when using longer needles (8mm+) in lean patients. For more detail on SubQ technique, see our Subcutaneous Injection Technique guide.

For subcutaneous testosterone (where your prescriber has directed the SubQ route), a 31G 6mm insulin syringe is the standard home-injection choice. Use pinch-up technique, especially if you have low body fat, to ensure the medication deposits in subcutaneous tissue rather than at the fat-muscle interface.

4. Needles for Intramuscular Injection

Intramuscular (IM) injections deliver medication directly into muscle tissue. This method is used for:

  • Testosterone (TRT, traditional IM protocols)
  • Vitamin B12 (hydroxocobalamin or cyanocobalamin, prescription-only in Australia) -- typically administered IM by a practice nurse. Self-injection is possible but always under prescriber direction.
  • Certain hormone therapies

Recommended gauge for injection: 23G to 25G
Recommended length: 25mm (1 inch) for deltoid and thigh; 38mm (1.5 inches) for gluteal

Important: IS does not stock IM-gauge needles (21G-25G, 25-38mm). If your prescriber has you on an intramuscular protocol, source these supplies through your pharmacy.

IM injections use thicker, longer needles because the medication must pass through the skin, subcutaneous fat, and into the muscle belly. Oil-based medications like testosterone cypionate or enanthate are viscous and flow slowly through thin needles, so a 23G-25G is the practical balance between comfort and injection speed.

Some older IM injection guides recommend a two-needle technique (draw with 18G, swap to a 22G-25G to inject). In current practice, many home injectors simply use the injection needle to draw and inject. Either approach can be clinically appropriate -- if your prescriber has specified a particular workflow, follow their guidance. Our store does not stock IM-gauge needles (22G-25G) -- source these from your pharmacy if your prescriber has you on an intramuscular protocol.

5. Drawing Needles

The main home use for an 18G draw syringe is peptide reconstitution -- specifically when transferring a larger volume (2ml or more) of bacteriostatic water into a lyophilised peptide vial in a single pass. Using a 1ml insulin syringe for the same job requires multiple transfers, and you should use a fresh insulin syringe for each transfer rather than re-puncturing the stopper with the same needle.

Recommended: An 18G draw syringe with 10ml syringe is the standard choice for peptide reconstitution. The 18G bore allows the bacteriostatic water to flow easily in a single pass, and the 10ml barrel is large enough for reconstitution work.

Do not inject with a drawing needle. An 18G needle is far too thick for injection and will cause unnecessary pain and tissue damage. It exists solely for transferring liquid between containers.

6. Needle Size by Medication Type

Here is a quick-reference table for the most common self-administered medications in Australia:

Medication Injection Type Injection Gauge Needle Length Draw Needle?
Compounded semaglutide SubQ 29G-31G 6mm No (use insulin syringe directly)
Tirzepatide (compounded) SubQ 29G-31G 6mm No
Insulin SubQ 29G-31G 4-6mm No
BPC-157 / TB-500 SubQ 29G-31G 6mm Separate draw recommended for reconstitution
Testosterone (SubQ, prescriber-directed) SubQ 31G 6mm Use pinch-up technique
Testosterone (IM protocol) IM 23G-25G 25-38mm Not stocked -- source from pharmacy
HCG SubQ 29G-31G 6mm Optional
B12 (IM) IM 23G-25G 25mm Not stocked -- source from pharmacy

If your medication is not listed above, the general rule is: water-based and thin solutions use fine needles (29G-31G, SubQ) while oil-based and viscous solutions used for IM injection use thicker needles (23G-25G). Your prescribing doctor or compounding pharmacy will specify the injection method.

Frequently Asked Questions

What gauge needle is used for subcutaneous injections?

For subcutaneous injection, 29G to 31G with a 4-8mm length is standard. The 31G is the thinnest commonly available and causes the least sensation on insertion. A 1ml 31G 6mm insulin syringe covers the vast majority of SubQ medications, including GLP-1s, peptides, insulin, and SubQ testosterone.

What gauge needle for testosterone?

It depends on the injection method. For subcutaneous testosterone (where your prescriber has directed the SubQ route), a 31G 6mm insulin syringe handles both drawing and injecting. Use pinch-up technique to ensure the medication deposits in subcutaneous tissue. For intramuscular testosterone, use a 23G-25G needle with 25-38mm length -- these are not stocked here, source from your pharmacy. Follow your prescriber's specific guidance.

Does a thinner needle hurt less?

Yes. Thinner needles (higher gauge numbers) displace less tissue during insertion and cause less discomfort. The difference between a 25G and a 31G needle is noticeable. However, thinner needles also flow more slowly, so there is a practical limit: injecting viscous oil-based testosterone through a 31G needle for SubQ takes 30-60 seconds, which is acceptable for SubQ doses. For thin, water-based solutions (semaglutide, peptides, insulin), the thinnest available needle (31G) is the best choice for comfort.

Can I use an insulin syringe for peptides?

Yes. Insulin syringes with 29G-31G needles are the standard choice for reconstituted peptides like BPC-157 and TB-500. The syringe scale is marked in units (100 units = 1ml), which makes precise small-volume dosing straightforward. See our Peptide Therapy Supplies guide for a detailed breakdown of what you need.

Why do I need a separate drawing needle?

For peptide reconstitution, an 18G draw syringe lets you transfer 2ml or more of bacteriostatic water into a peptide vial in a single pass -- much faster than multiple insulin-syringe transfers. It also avoids the need to repeatedly puncture the peptide vial stopper with a fine needle (which dulls faster and can cause rubber coring). For SubQ testosterone, drawing directly with the insulin syringe is standard practice -- the 30-60 second draw time through a 31G needle is acceptable for the small doses involved.

What length needle do I need?

For subcutaneous injection: 4-8mm. The 6mm length works for most adults and body types. With a 4mm or 6mm needle, 90-degree injection into a pinched skin fold is appropriate for most adults including lean individuals. 45-degree angle is recommended with longer needles (8mm+) in lean patients. For intramuscular injection: 25mm (1 inch) for the deltoid or thigh, 38mm (1.5 inches) for the gluteal. IM needles are not stocked here -- source from your pharmacy.

Disclaimer: This article is for informational purposes only and does not constitute medical advice. Always consult a qualified healthcare professional before starting any injection therapy, changing your medication routine, or if you have any health concerns.
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