An insulin syringe is calibrated in units, not mL. If you’ve never used one, the scale looks wrong and the maths feels backwards. It isn’t — but it does need explaining once, properly.
What “U-100” means
U-100 is an insulin-industry convention. It means the syringe is calibrated for insulin at a concentration of 100 units per mL. So:
- 1 unit = 0.01 mL
- 10 units = 0.1 mL
- 50 units = 0.5 mL
- 100 units = 1 mL (full barrel)
That conversion is the entire trick. The “units” scale on the barrel is just a more convenient way to read very small volumes. A 0.01 mL line on a regular syringe is illegible; a “1 unit” line is easy.
For research peptides — which usually aren’t insulin — we ignore what U-100 was designed for and just treat the syringe as a precise small-volume measuring tool. The unit number itself doesn’t tell you anything about peptide dose; you need to combine it with the vial’s concentration.
How to read the barrel
Read from the front edge of the plunger. Not the rear edge, not the middle of the rubber. Most insulin syringes have a clear flat front face on the rubber tip — that’s the reading line.
Units → mg conversion
Once you know the units, the mg dose depends on the vial’s concentration:
mg drawn = units × 0.01 × concentration (mg/mL)
Examples
| Vial concentration | Draw 10 units | Draw 20 units | Draw 30 units |
|---|---|---|---|
| 1 mg/mL | 0.1 mg (100 µg) | 0.2 mg (200 µg) | 0.3 mg (300 µg) |
| 2 mg/mL | 0.2 mg (200 µg) | 0.4 mg (400 µg) | 0.6 mg (600 µg) |
| 2.5 mg/mL | 0.25 mg (250 µg) | 0.5 mg (500 µg) | 0.75 mg (750 µg) |
| 5 mg/mL | 0.5 mg (500 µg) | 1.0 mg | 1.5 mg |
Picking gauge and length
Two things to choose: how thick the needle is (gauge, written as G) and how long it is.
Gauge
- 29G–31G — standard for subcutaneous injection. Thin, comfortable, minimal trauma.
- 27G–28G — for thicker or more viscous solutions, or for intramuscular use. More discomfort, faster flow.
- 25G or lower — generally too thick for research peptide subcutaneous work. Used in venous draws or larger transfers.
Higher number = thinner needle. 31G is finer than 27G.
Length
- 4 mm – 6 mm (3/16″ – 1/4″) — short. Standard for subcutaneous injection into the abdomen or thigh. Enough to clear the skin without hitting muscle.
- 8 mm – 13 mm (5/16″ – 1/2″) — medium. Used with a pinched skin fold or for people with more subcutaneous tissue.
- 25 mm+ (1″ or more) — long. Intramuscular only. Not for SubQ peptide work.
Default starting choice for most research peptides: 30G or 31G, 5 mm or 8 mm length, in a U-100 1 mL syringe.
Drawing technique
- Wipe the vial stopper with an alcohol swab. Let it air-dry.
- Pull the plunger to the unit mark of the dose you want — yes, you draw air into the syringe first.
- Insert the needle through the stopper. Push the plunger down to inject the air into the vial. This equalises pressure so the liquid draws out smoothly later.
- Invert the vial so the liquid covers the needle tip. Pull the plunger slowly to the unit mark you want.
- Tap the barrel to lift any air bubbles to the top, then push them back into the vial. Re-draw if needed to land exactly on the unit line.
- Withdraw the needle. Keep the syringe pointing up until you’re ready to inject.