U-40 vs U-100 Syringe: Why the Unit Marks Differ

What the U number means

On a U-100 syringe, 100 units fill 1 mL^2,3. On a U-40 syringe, 40 units fill 1 mL¹. So a unit on the barrel is a share of 1 mL, and its size depends on the U number.

Most insulin syringes are marked for U-100³. U-40, also written U40, is printed on the veterinary insulin label we read¹.

Same mL, different numbers

The math takes one step. Divide units by the U number to get mL. Multiply mL by the U number to get units.

Example readings to show the math, not an amount to take.

For the same volume, a U-100 scale always shows 2.5 times the number a U-40 scale shows. If your syringe is marked in mL or cc instead, see how to read an insulin syringe.

Match the syringe to the label

The U-40 label we read says to use its insulin with U-40 syringes only, because any other syringe will result in incorrect dosing¹. ADA says the same for U-100: the syringe must be made for the same concentration as the vial². If your label and your syringe show different U numbers, stop and ask your pharmacist or vet.

The reconstitution calculator has a U-40, 1 mL option and shows units and mL together. If you switch between U-100 and U-40, it notes that the same volume reads as different units. Open the reconstitution calculator to check your own numbers.

Read the barrel you have

Count the lines on your own barrel; the printed numbers tell you the step. For a U-100 barrel, see how to read an insulin syringe and the printable syringe chart. For mg, use mg to units.