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Syringe Units Explained: U-100, mL, mg, mcg and IU
The marks on an insulin syringe are not milligrams and not micrograms. They are a way to measure a volume of liquid. To get from a dose to a mark you convert twice: dose to volume, then volume to units.
What the markings mean
Insulin syringes are labelled with the strength of insulin they were designed for. A U-100 syringe holds 100 units per millilitre, so one unit is 0.01 mL. This is the standard in the United States and most peptide protocols assume it.
U-40 syringes (used in veterinary medicine in some countries) hold 40 units per mL, so one unit is 0.025 mL. Using a U-40 syringe with a U-100 calculation draws 2.5 times too much liquid. Check the barrel before you trust any chart.
| Syringe | Units per mL | Volume of 1 unit | Common sizes |
|---|---|---|---|
| U-100 | 100 | 0.01 mL | 0.3 mL (30 units), 0.5 mL (50 units), 1 mL (100 units) |
| U-50 | 50 | 0.02 mL | Rare; mostly older stock |
| U-40 | 40 | 0.025 mL | 1 mL (40 units), mainly veterinary |
Two conversions
- Dose to volume: volume (mL) = dose (mg) ÷ concentration (mg/mL). Convert mcg to mg by dividing by 1,000.
- Volume to units: units = volume (mL) × 100 on a U-100 syringe.
Shortcut on a U-100 syringe: units = dose (mcg) ÷ [concentration (mg/mL) × 10]. For 250 mcg at 2.5 mg/mL that is 250 ÷ 25 = 10 units.
Quick table: units for common doses at 2.5 mg/mL
A dose of 100 mcg here is only 4 units, which is too small to measure well. Adding more water to the vial would make the same dose easier to read.
| Dose | Volume | U-100 units |
|---|---|---|
| 100 mcg | 0.04 mL | 4 |
| 250 mcg | 0.10 mL | 10 |
| 500 mcg | 0.20 mL | 20 |
| 1 mg | 0.40 mL | 40 |
| 2 mg | 0.80 mL | 80 |
Units, IU, mg and mcg are four different things
- mg and mcg measure mass: how much peptide there is.
- mL measures volume: how much liquid.
- Syringe units are a volume scale, 0.01 mL each on U-100.
- IU (international units) measures biological activity and is used for some hormones such as hCG or growth hormone. An IU dose still has to be converted to a volume using the vial label.
Checking your work
- Say the arithmetic out loud once: vial mg, water mL, concentration, dose, volume, units.
- Ask whether the answer is reasonable. A routine dose that comes out at 150 units, or at 1 unit, is a sign of a slip.
- Compare with a calculator or the syringe simulator, then choose the one you trust and stay consistent.
Common questions
Does a 0.3 mL syringe change the maths?
No. It is still U-100 if so marked. It simply stops at 30 units, and its marks are usually finer, which helps with small doses.
What if my dose is more than 100 units?
It will not fit in a 1 mL U-100 syringe. Reconstitute with less water to raise the concentration, or split the dose into two injections.
How many decimal places should I trust?
On a standard syringe, about half a unit at best. Round sensibly and design the water volume so the dose lands on a clear mark.
Try it
- Syringe Simulator: See the plunger line on a syringe, then test yourself with practice problems.
- Dosage Calculator: Vial size, water and dose in; mL and syringe units out, plus doses per vial.