Syringes and needles, translated
Three numbers describe every syringe: gauge, which runs backwards so a bigger number means a thinner needle; units, where 100 units is 1 mL on any U-100 barrel; and barrel size, which decides how far apart the printed lines sit. Here is what each one means, and which combinations people actually end up buying.
Last edited · written by PepPrep · not medical advice
Three numbers describe every syringe, and none of them are intuitive:
- Gauge (G) is needle thickness, and it runs backwards: a bigger number means a thinner needle. A 31G needle is hair-thin; an 18G is thick (and only used for drawing liquid, not injecting).
- Units are the markings on insulin syringes. On every standard "U-100" syringe, 100 units = 1 mL. So 10 units is 0.1 mL, no matter which barrel size you own.
- Barrel size (0.3 / 0.5 / 1.0 mL) is how much it holds. Smaller barrels put more space between the lines, which makes tiny doses easier to read accurately.
| Syringe | Marked | Needle | Best for |
|---|---|---|---|
| 0.3 mL insulin | 0–30 units | 31G, very thin | Tiny doses — widest spacing between lines |
| 0.5 mL insulin | 0–50 units | 31G, very thin | The everyday all-rounder most people use |
| 1.0 mL insulin | 0–100 units | 30G, thin | Larger doses that won't fit the small barrels |
| 3 mL luer-lock | mL markings | Screw-on (18G blunt) | Mixing and transferring between vials |
What most people buy: 31G, 0.5 mL insulin syringes for everyday small doses; a box of 3 mL luer-lock syringes with 18G blunt fill needles for mixing and transferring; and the 0.3 mL size if their doses turn out tiny.
One more branch of the family: pen needles. If your product comes in a pre-filled injection pen rather than a vial, you don't need syringes at all — just screw-on pen needles (32G, 4 mm is the popular pick) and the same alcohol pads and sharps container as everyone else. Pens vs vials and syringes puts the two shopping lists side by side.
"Can I reuse a syringe?"
This gets asked constantly and usually gets answered badly — either a flat lecture, or a shrug. Here is what's actually true.
Every manufacturer says single-use, and health bodies follow them. That's the formal answer and it's the one to follow. What's more interesting is that the published evidence is genuinely thin: reuse has been studied for decades without a clear verdict either way. So the honest position is that nobody is hiding proof from you — but "unproven" is not the same as "fine", and the people who make the things still say once.
What is not in dispute is the mechanical side, and that's the part this site can speak to:
- Needles blunt on first use. Under magnification a used needle tip is visibly deformed. That's felt as more sting and more bruising, and it's the biggest single reason a second injection hurts more than the first.
- The risk isn't only to you — it's to the vial. A needle that's been in skin and then goes back into a multi-dose vial can carry contamination into liquid you'll be drawing from for weeks. Reusing a syringe on a shared vial risks the whole vial, not just one injection.
- Markings wear off. Printed scales rub away with handling, and a syringe you can't read accurately is worse than no syringe.
- Recapping is where injuries happen. Anything that involves putting the cap back on a used needle by hand adds the most common needlestick scenario to your routine.
The reason people reuse is almost always cost — and that has a better answer. Bought singly, syringes feel expensive. Bought in a 100-count box, they come out at pennies each, which is usually less than the value of the liquid wasted in one bad measurement. If cost is the pressure, buying the bigger box solves it more cleanly than reuse does.
If reuse is a genuine constraint rather than a convenience, that's a conversation for a clinician or a pharmacist, who can weigh your actual situation. It isn't something to settle from a comment thread — or from a shop.
Common questions
What does 31G mean?
Gauge: needle thickness. Higher is thinner, so 31G is thinner than 30G. See the gauge term page.
Is 31G thinner than 30G?
Yes, by one step. 32G is thinner still; most people notice a small difference, not a large one.
How many units does a 0.3 mL syringe hold?
30 units on the U-100 scale; a 0.5 mL holds 50 and a 1 mL holds 100.
What is a U-100 syringe?
One marked at 100 units to the millilitre, so a unit is 0.01 mL. U-40 syringes exist for animals and read differently; check the barrel.
Is 0.3 mL or 0.5 mL better?
0.3 mL if every draw is under 30 units (wider spacing, half-unit versions exist); 0.5 mL if draws vary or reach 50. See the comparison.
What are half-unit markings?
Lines every 0.5 unit (0.005 mL) on a 0.3 mL barrel, for reading very small amounts.
Supplies mentioned: Syringes & Needles · reconstitution calculator · Which one do I need?
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.