How to read insulin syringe units

A unit is a volume

On a U-100 insulin syringe — the standard type — the barrel is divided into 100 units per millilitre. So:

• 1 unit = 0.01 mL
• 10 units = 0.1 mL
• 100 units = 1 mL (a full 1 mL syringe)

A unit says nothing about how many milligrams you are drawing. That depends entirely on how much water you put in the vial. The same 10 units can be 0.4 mg or 4 mg depending on the mix — which is why the water volume has to be decided before anything else.

The three common barrel sizes

• 0.3 mL — 30 units. Finest markings, best for small draws. Every unit is printed.
• 0.5 mL — 50 units. The middle option.
• 1 mL — 100 units. Most volume, coarsest markings — often numbered every 10.

A smaller barrel is not a smaller dose. It is a more readable one. If your answer lands at 4 units on a 1 mL syringe, the same draw is much easier to see on a 0.3 mL.

U-40 syringes are a different scale

A U-40 syringe has 40 units per millilitre, so one unit is 0.025 mL — two and a half times the volume of a U-100 unit. Drawing to "20 units" on a U-40 syringe when your number was calculated for U-100 gives you two and a half times the intended dose.

Check the barrel before you draw. The calculator asks which syringe you have for exactly this reason.

Reading the mark

Line up the flat top edge of the plunger stopper with the mark, not the tip of the rubber cone. Read at eye level; looking down at an angle shifts the reading.

Our calculator prefers answers that land on a printed line rather than between two of them — when it finds one, it says "lands on a printed line" under the barrel.