⚠ Critical dose-safety warning -- U-100 only

This article covers standard U-100 insulin syringes (100 units per ml). If your vial is labelled U-200, U-300, U-500, or anything other than U-100, STOP. For non-U-100 insulin, the syringe marks no longer correspond to the insulin units in the vial. They still represent volume, but dosing concentrated insulin requires the device or mL conversion supplied in your written dose directions. Drawing to the 20-unit mark on a U-100 syringe from a U-500 vial would draw 0.2ml, which contains 100 units of U-500 insulin -- five times a 20-unit dose. Ask for help before drawing up any dose from a concentrated insulin vial.

When you are new to vial-drawn injections, one of the first practical questions is: which syringe do I actually need? The answer depends on the labelled draw volume, the medicine route, and the written directions you were given.

Once you understand the scale, the next step is matching it to the labelled draw volume and device directions you have been given. This guide shows how syringe specifications relate to that information.

We'll walk through the difference between 0.5ml and 1ml U-100 syringes, explain needle gauges, and show how to choose a barrel size once your label gives the volume to draw. For prescribed subcutaneous (under the skin) products, compounded products, or testosterone, confirm the exact equipment before use.

Comparison of 0.5ml and 1ml insulin syringes flat-lay
0.5ml vs 1ml insulin syringes -- both 31G 6mm, different barrel sizes.

Anatomy of an Insulin Syringe

Before we get into insulin syringe sizes, it helps to understand what you're looking at. A standard insulin syringe has three parts:

  • Barrel -- the clear cylinder that holds the medication. The graduated markings on the side tell you how much liquid is inside.
  • Plunger -- the rubber-tipped rod you push to inject. Pull it back to draw up medication, push it forward to inject.
  • Needle -- attached to the front. On these insulin syringes, the needle is permanently fixed (not detachable). Many insulin syringes are fixed-needle, but always check the actual device you are using. The gauge number tells you how thick it is; the length tells you how deep it penetrates.
Diagram showing parts of an insulin syringe: barrel, plunger, needle
The three main parts of an insulin syringe.

The markings on the barrel are the most important thing to understand. On U-100 insulin syringes, the numbered unit marks are volume marks: 1 unit = 0.01ml, 10 units = 0.10ml, and 100 units = 1.0ml. A 0.5ml syringe holds 50 units and a 1ml syringe holds 100 units. Check the markings on the actual syringe you are using before drawing up a dose.

That last point matters more than most people realise, and we'll come back to it.

Insulin Syringe Sizes: 0.5ml vs 1ml -- What's the Difference?

The core difference between insulin syringe sizes is straightforward: volume capacity. A 0.5ml syringe holds up to 0.5ml of fluid. A 1ml syringe holds up to 1ml.

But that's only half the story. The real question is accuracy.

When you're drawing up a small volume -- say, 0.2ml -- that is the 20-unit mark on any U-100 insulin syringe. A 0.5ml syringe can be easier to read for small volumes because the usable range is limited to 0--50 units, but the unit-to-ml conversion is the same as on a 1ml syringe.

Volume markings comparison between 0.5ml and 1ml insulin syringes
The same 0.2ml dose is easier to read accurately on the 0.5ml syringe (left) than the 1ml (right).

Here's a quick comparison:

Feature 0.5ml Syringe 1ml Syringe
Max volume 0.5ml (50 units) 1ml (100 units)
Best for doses Up to 0.5ml 0.5ml--1ml (or when 0.5ml is unavailable)
Readability at small volumes Often easier -- markings are visually spread out Less visual spacing near the bottom of the barrel
Common uses Small labelled draw volumes, including small insulin doses and some prescribed vial-drawn medicines Labelled draw volumes over 0.5ml and up to 1ml, if your instructions confirm a U-100 insulin syringe is suitable
Price $34.95 / 100 pack $34.95 / 100 pack

Both our 0.5ml insulin syringes and 1ml insulin syringes use the same 31G 6mm needle -- so the main difference is barrel capacity and how easy the markings are to read for your draw volume.

Needle Gauge and Length Explained

The needle spec you'll see on our syringes is 31G 6mm. Here's what that means:

Gauge (G) -- Thickness

Gauge is counterintuitive: the higher the number, the thinner the needle. Common gauges for insulin-style injections:

  • 29G -- still widely used in insulin pen needles and some syringes.
  • 30G -- a thin gauge used for some low-viscosity subcutaneous injections when the product instructions or clinician confirm it is suitable.
  • 31G -- a very fine option commonly used for many low-viscosity subcutaneous injections, but suitability depends on the medicine, route, volume, site, and prescriber or pharmacist instructions.

For many low-viscosity aqueous subcutaneous (under the skin) injections, 31G may be suitable when confirmed by the prescriber, pharmacist, or product instructions. Follow your prescriber, pharmacist, or product instructions for the exact gauge and length.

Length -- Depth of Penetration

Length tells you how deep the needle goes. For subcutaneous injections, you want to reach the fat layer just below the skin -- not muscle, not surface tissue.

  • 4mm -- commonly used for subcutaneous insulin delivery in many adults; suitability still depends on product instructions, site, body composition, and technique
  • 6mm -- commonly used for subcutaneous injection in many adults. Site, body composition, insertion angle, and pinch-up technique still matter.
  • 8mm -- longer than most short insulin needles and may require a pinch-up or angled technique to reduce intramuscular (into the muscle beneath the fat) risk. True intramuscular injection usually requires a longer needle selected for the site and body composition.

Our 6mm needles are commonly used for abdominal subcutaneous injections when the medicine label or prescriber supports that route. In very lean people or thin injection sites, angle and pinch-up technique can affect whether the needle stays in subcutaneous tissue.

Which Syringe for Which Medication

This is the practical section. Let's go through the most common scenarios.

Prescribed vial-drawn medicines

If your prescriber or pharmacist has supplied a vial and your labelled draw volume is 0.5ml or less, a 0.5ml syringe may be suitable only if your instructions confirm a fixed-needle U-100 insulin syringe is appropriate for the route, viscosity, vial type, and draw volume. For draw volumes over 0.5ml and up to 1ml, a 1ml syringe may be suitable if your instructions confirm it. If the volume is over 1ml, ask your prescriber or pharmacist for the correct device and injection plan. Do not choose a syringe from the milligram dose alone; concentration determines volume.

For a broader supplies checklist for prescribed vial-drawn subcutaneous medicines, see our vial-drawn subcutaneous injection supplies guide.

For a full injection technique walkthrough, see our subcutaneous injection technique guide.

Insulin

Insulin dosing is measured in units (U), not millilitres. Standard insulin syringes are calibrated for U-100 insulin (100 units per ml):

  • 0.5ml syringe = 0--50 units
  • 1ml syringe = 0--100 units

If your insulin dose is 50 units or under, either syringe may work only when the exact prescribed dose aligns with the visible graduations on that syringe, but many people find the 0.5ml syringe easier to read for smaller doses because the markings are visually more spread out. For small, odd-numbered, or half-unit doses, use the smaller-capacity syringe, half-unit syringe, pen, or other device specified by your prescriber or pharmacist. For U-100 insulin doses over 50 units and up to 100 units, a 1ml U-100 syringe has the required capacity unless your clinician has specified another device or injection plan. For doses above 100 units or any unusual insulin plan, follow your clinician's device instructions.

Insulin dose miscalculation can cause severe hypoglycaemia (low blood sugar), alongside factors such as food intake, exercise, illness, timing, and other medicines. Double-check your draw against your prescribed dose before every injection.

Prescribed reconstituted products

Some prescribed reconstituted products use small subcutaneous draw volumes, but the diluent, concentration, route, and device must come from your prescriber or pharmacist. A few general rules:

  • For any prescribed reconstituted product, use the dose volume and route supplied by your prescriber or pharmacist. If that draw volume is 0.5ml or less, a 0.5ml syringe may be suitable only if your prescriber or pharmacist confirms a fixed-needle U-100 insulin syringe is appropriate for that product, route, viscosity, vial type, and draw volume.
  • Calculate the per-dose volume only from your prescribed dose and the labelled concentration or pharmacist-specified reconstitution details. Use a new sterile syringe and needle, or a new fixed-needle syringe, for each injection and any separate vial entry unless your pharmacist has given specific reconstitution instructions. Changing only the needle does not make a used syringe sterile.
  • If your draw volume is over 0.5ml and up to 1ml, use the 1ml syringe if your instructions confirm this device is suitable. If it is over 1ml, ask your prescriber or pharmacist what device and injection plan to use.

Insulin syringes are intended for subcutaneous use. If a medication is prescribed intramuscularly, ask your prescriber or pharmacist which needle length and gauge are appropriate for the site and your body composition.

TRT (Testosterone Replacement Therapy)

Testosterone is typically a thicker oil-based compound, which is where the syringe setup gets a bit different.

Note on TRT administration route: Australian TGA-approved testosterone products (Reandron 1000, Primoteston Depot, Sustanon 250) are licensed for intramuscular injection. Some clinicians may direct subcutaneous administration of specific testosterone products in individual circumstances -- this is an off-label prescribing decision, not a DIY choice. Always follow your prescriber's specific route and dose instructions. If unsure, ask your prescriber or compounding pharmacist.

⚠ Reandron (testosterone undecanoate 1000mg/4mL): This product is a viscous oil licensed for deep intramuscular gluteal injection only. Do not use a fixed 31G 6mm insulin syringe for Reandron. It is not suitable for slow deep intramuscular gluteal administration, and the oil viscosity and 4ml volume require clinician-selected equipment. Reandron is typically administered by a GP or practice nurse. Do not attempt to self-administer Reandron subcutaneously.

For prescriber-directed subcutaneous testosterone, confirm with your prescriber or compounding pharmacist whether a fixed 31G 6mm insulin syringe is appropriate for both drawing and injecting that specific formulation. Oil viscosity, dose volume, vial type, and route all affect the safest equipment choice.

When to call your prescriber

Most injections go smoothly - a small drop of blood or a brief sting are normal. The list below covers things that aren't normal and warrant a phone call to your prescriber:

  • Spreading or worsening redness, warmth, swelling, pus, red streaking, severe pain, or fever at any time after injection
  • A hard lump that doesn't go away after a week, or one that's hot and tender
  • Fever, chills or feeling unwell after an injection
  • Any rash, hives or itching at or beyond the injection site (possible allergic reaction)
  • Signs of a severe allergic reaction - swelling of face or throat, difficulty breathing - call 000 immediately
  • You realise you injected from a vial stored outside the labelled temperature/time limits, past its in-use period, or from liquid that was unexpectedly cloudy or contained particles

Frequently Asked Questions

What's the difference between 0.5ml and 1ml insulin syringes?

The difference is volume capacity and readability. A 0.5ml U-100 syringe holds up to 0.5ml (50 units). A 1ml U-100 syringe holds up to 1ml (100 units) and may be suitable for draw volumes over 0.5ml and up to 1ml if your instructions confirm it. If the volume is over 1ml, ask your prescriber or pharmacist for the correct device and injection plan. For small volumes under 0.5ml, many people find the 0.5ml syringe easier to read. Both products use the same 31G 6mm needle.

What gauge needle should I use for subcutaneous injections?

31G may be suitable for some low-viscosity aqueous subcutaneous injections when confirmed by the prescriber, pharmacist, or product instructions, but use the gauge and length specified for your product. Flow, viscosity, route, site, and body composition all matter. For compounded medicines such as prescriber-directed subcutaneous testosterone, confirm whether a fixed 31G 6mm insulin syringe is appropriate for that exact formulation. Intramuscular testosterone requires a longer, thicker needle selected under clinical guidance.

Can I use an insulin syringe for peptides?

Use injection equipment only for prescribed products. Do not use this guidance for non-prescribed or online peptide products; many peptides are not TGA-approved therapeutic goods in Australia.

If your prescriber or pharmacist has instructed you to draw a subcutaneous dose from a vial, they should specify the dose volume, route, and suitable device. If they confirm a fixed-needle U-100 insulin syringe is appropriate, choose 0.5ml for confirmed draw volumes at or below 0.5ml, or 1ml for confirmed draw volumes over 0.5ml and up to 1ml. If the volume is over 1ml, ask your prescriber or pharmacist for the correct device and injection plan. Do not infer dose volume from online protocol examples.

How do I know which syringe size to buy?

Work backwards from your dose volume:

  1. Prefer the labelled draw volume in ml from your prescriber or pharmacist. Do not self-calculate an injection volume unless they have given you the dose, concentration, units, and route in writing. IU or medicine "units" are not the same as U-100 syringe units.
  2. Use the final concentration and draw volume provided in your prescriber or pharmacist instructions. Do not rely on a vial label alone for reconstituted products.
  3. Only divide when the dose and concentration are in the same medicine units, for example mg divided by mg/ml or IU divided by IU/ml. Do not convert between mg, micrograms, IU, or medicine-specific units unless your prescriber or pharmacist has given the exact conversion.
  4. Use the calculation only as a check when your written instructions provide the dose, concentration, units, route, and device suitability.
  5. Match the calculated volume to the prescriber- or pharmacist-approved device; do not use volume alone to choose an insulin syringe. If the volume is over 1ml, ask your prescriber or pharmacist for the right device and injection plan.

When in doubt, ask your prescriber or pharmacist for the exact draw volume in millilitres. If the draw volume is 0.5ml or less, the 0.5ml syringe is often easier to read. If it is over 0.5ml and up to 1ml, use the 1ml syringe or the device specified on your label. If it is over 1ml, get device and injection-plan instructions before injecting.

Use the ml draw volume provided by your prescriber or pharmacist. If they have given the dose, concentration, units, route, and device instructions in writing, you can use the volume calculation only as a check; otherwise ask them for the exact ml draw volume. Do not read medicine IU or protocol "units" as the same thing as the unit markings on a U-100 syringe.

Where is 0.2 mL on an insulin syringe?

On a U-100 1 mL insulin syringe, 0.2 mL is the 20-unit mark (1 mL = 100 units, so each 0.1 mL = 10 units). On a 0.5 mL U-100 syringe, 0.2 mL is the 20-unit mark as well - the unit scale is identical; the difference is just the maximum capacity. Use our dose volume calculator only to translate a prescriber- or pharmacist-confirmed mL draw volume to the matching mark on a U-100 syringe, after confirming that a U-100 insulin syringe is suitable for that product, route, and volume.

Where is 0.5 mL on a 1 mL syringe?

0.5 mL is the 50-unit line on a U-100 1 mL insulin syringe. If your dose is 0.5 mL or less, a 0.5 mL syringe may be easier to read because the usable range is visually more spread out; check the graduation increments on the actual syringe. If the draw volume is above 0.5 mL and up to 1 mL, use a 1 mL syringe if your instructions confirm it is suitable.

How many units is 0.1 mL?

0.1 mL = 10 units on a standard U-100 insulin syringe. The conversion is 1 mL = 100 units regardless of barrel size, so every 0.01 mL = 1 unit on the scale.

How many units is 0.25 mL?

0.25 mL = 25 units on a U-100 insulin syringe. This volume sits at the quarter mark on a 1 mL barrel (between the 20 and 30 unit lines) or at the halfway point on a 0.5 mL barrel.

Help me pick a syringe for my exact dose

Use the dose volume calculator only to check a prescriber- or pharmacist-supplied mL draw volume, or when your written instructions state the dose, final concentration, units, route, and suitable device. Do not calculate from vial labels or online protocols alone.

Which Syringe Should You Order?

Here's the short version:

  • Lawfully supplied prescribed vial with draw volume 0.5ml or under (confirm the volume and device on your label): 0.5ml syringes may be suitable only if your prescriber or pharmacist confirms a fixed-needle U-100 insulin syringe is appropriate for that product's route, viscosity, vial type and draw volume; otherwise use the device specified by your prescriber or pharmacist
  • Insulin 50 units or under: either may work; a 0.5ml syringe is often easier to read for smaller doses
  • U-100 insulin over 50 units and up to 100 units: 1ml syringes have the required capacity unless your clinician has specified another device or plan
  • Prescribed vial-drawn products with draw volume 0.5ml or under: 0.5ml syringes may be suitable only if your prescriber or pharmacist confirms a fixed-needle U-100 insulin syringe is appropriate for that product's route, viscosity, vial type and draw volume; otherwise use the device specified by your prescriber or pharmacist
  • For prescriber-directed subcutaneous testosterone: ask your prescriber or compounding pharmacist whether a fixed-needle insulin syringe is appropriate, and which barrel size matches the exact formulation and volume.
  • If both sizes have been confirmed suitable and you want flexibility: order both. If you are unsure about dose volume, route, or device suitability, ask your prescriber or pharmacist first.

Both syringe packs come with 100 syringes, 31G 6mm fixed needles, and are dispatched same day for most metro orders. If you're running low or setting up for the first time, add a pack of alcohol prep wipes to your order.

For dosing, route, or protocol questions, ask your prescriber or pharmacist. We can help match syringe capacity to the labelled draw volume and equipment instructions you have been given.