Reading a U-100 syringe

Updated October 2026 · Educational math explainer, not medical advice.

The numbers below are made-up examples to show the arithmetic. They are not dose suggestions. Always use the dose your prescriber gave you and the instructions from your pharmacist.

Pulling the plunger back to the 40-unit mark draws 0.40 mL (100 units = 1 mL). Read at the top edge of the rubber seal.

"U-100" describes the syringe's scale: 100 units = 1 mL, so 1 unit = 0.01 mL. These are commonly called insulin syringes, but the scale is just a volume scale.

Converting

mL → units: multiply by 100 (0.25 mL = 25 units)

units → mL: divide by 100 (40 units = 0.40 mL)

Common sizes and markings

Syringe	Holds	Typical tick marks
30-unit	0.3 mL	often every 0.5 or 1 unit
50-unit	0.5 mL	usually every 1 unit
100-unit	1 mL	usually every 2 units

Markings vary by brand, so check the printing on your syringe box. VialMath shows the exact value and the nearest tick for the size you pick, clearly labeled as rounded.

Why "units" here don't mean a drug amount

Syringe units measure volume, not how much medication you get. 10 units of a 2.5 mg/mL solution and 10 units of a 5 mg/mL solution are very different amounts. That's why the concentration has to be right first.

Not the same as "IU"

International units (IU) on a medication label are a measure of biological activity and differ by substance. VialMath doesn't convert IU automatically.

Precision tips

Read the scale at the top edge of the plunger's rubber seal.
Small draws (under about 2 units) are hard to measure accurately. Ask your pharmacist if that's what your math says.
If a draw is bigger than your syringe holds, stop and ask; don't guess a split.

Try it with your own numbers: open the calculator. It shows every step so you can check the math.

This tool does arithmetic on numbers you enter. It does not recommend doses. Confirm with your prescriber or pharmacist.