Insulin Syringe Units Explained: How to Read U100 & U40 Markings

The standard U100 insulin syringe is marked with 100 unit divisions with each unit being 1/100 milliliter. Most U100 insulin syringes come in three sizes: 0.3 ml (30 units), 0.5 ml (50 units), and 1 ml (100 units). The small lines on the barrel are numbered at 1 or 2 units (size of syringe). Hold the syringe at eye level to read the syringe; The flat top of the plunger indicates the reading. Checkout this GLP-1 dose calculator for better insights.

What Is a U100 Insulin Syringe?

A U100 insulin syringe is a medical syringe that is smaller and thinner, and is marked with a 100 unit per milliliter dosage. The "U" signifies "unit" and the "100" is the concentration that the syringe was calibrated to. In the United States, U100 insulin syringes are typically used for almost all compounded GLP-1 drugs.

If you use insulin, here's what you need to know about reading insulin syringe units. U100 syringes come in three common sizes: 0.3 ml (up to 30 units), 0.5 ml (up to 50 units), and 1 ml (up to 100 units). On the barrel of each syringe, each small line is 1 or 2 units. Follow the CDC's advice for safe injection: Hold the syringe vertically at eye level to read your dose. The U40 syringe contains 40 units per ml, which is unusual in the United States, and should never be used to compound GLP-1.

The smaller syringes have larger spaces between the markings, making them easier to read when administering small doses of medication. If you need more information about the volume math, refer to how many ml are in a volume.

The 3 Sizes of U100 Insulin Syringes

There are three main sizes of U100 insulin syringes, each accommodating a different insulin dose range.

0.3 ml (30 unit) syringe. Holds up to 30 units of liquid. Markings are typically at 1 unit intervals. For doses < 30 units best. It has the widest spacing between lines and is the most accurate for small doses of compounded GLP-1 (5 – 25 units).

0.5 ml (50 unit) syringe. Capable of holding up to 50 units of liquid. Markings tend to be in units of 1. Optimum dose is between 25 and 50 units. Typically, a dose of compounded tirzepatide is about 5 mg (50 units at 10 mg/ml).

1 ml (100 unit) syringe. Capacities of 100 units of liquid. Generally, markings come in twos. Most suitable for doses greater than 50 units. Required for larger doses of compounded tirzepatide.

If the dose is close to the upper limit of a syringe, stick to the next size up syringe. If you use a 30 unit syringe for the drawing, there is no room for an air bubble correction. The size of the syringe (0.5 ml or 0.3 ml) can make it easier or harder to draw your dose.

How to Read the Unit Markings

The insulin syringe units are marked along the barrel of the syringe. Each numbered line represents a multiple of 5 or 10 units. The unnumbered lines in between are individual units.

On a 0.3 ml or 0.5 ml U100 insulin syringe:

• Each small line = 1 unit

• Each numbered line = usually every 5 units (5, 10, 15, 20...)

On a 1 ml U100 insulin syringe:

• Each small line = 2 units

• Each numbered line = every 10 units (10, 20, 30...)

To read your dose:

1. Hold the syringe vertical at eye level

2. Look at where the flat top of the plunger meets the barrel markings (not the rounded tip)

3. Read the unit number where the plunger sits

Reading from below the barrel can lead to misreading by 1 to 2 units, which matters for small doses.

U40 Syringes: When They Exist and Why You Should Not Use Them

U40 insulin syringes have a dosage of 40 units per 1 ml as opposed to 100 units per 1 ml. Other uses of these in other countries and in veterinary medicine still occur. According to the American Diabetes Association, U100 is the standard for human insulin in the US, and is used for compounded GLP-1.

The compounded GLP-1 vial was meant for use on a U100 syringe, so if used on a U40 syringe, the dose will be incorrect by 2.5x. A "50 unit" mark on a U40 syringe indicates 1.25 mL of volume, or not 0.5 mL. You could end up taking too much if you draw the dose you think you should take.

Insulin syringe must always be labeled as "U-100" before using it for compounded GLP-1. It will be marked with the numbers U-100 and the orange cap will be the standard for the US.

Drawing Up a Dose: Step-by-Step

The basic technique for drawing a dose with a 1 ml syringe or smaller insulin syringe:

1. Wash your hands and clean the vial top with an alcohol swab.

2. Pull back the plunger to draw air equal to your dose. For 50 units, draw 50 units of air.

3. Insert the needle into the vial top and push the air into the vial.

4. Turn the vial upside down with the needle still in.

5. Pull the plunger back to slightly past your dose.

6. Tap the syringe to move air bubbles to the top.

7. Push slightly forward to expel air bubbles, leaving exactly your dose in the syringe.

8. Remove the needle from the vial.

9. Inject as directed by your prescriber.

Pushing air into the vial first prevents a vacuum that makes drawing harder.

Common Reading Errors and How to Avoid Them

Misreading 1-unit lines for 2-unit lines. On a 1 ml syringe, each line is 2 units, not 1. Mistakenly counting each line as 1 unit doubles your dose error.

Reading the rounded tip of the plunger. Use the flat top, not the curved bottom of the rubber stopper.

Drawing past your dose then pushing back. Pushing the plunger forward can introduce air or cause loss of medication. Draw slowly and carefully to your exact dose.

Confusing U100 with a regular medical syringe. Generic 1 ml syringes are marked in tenths of a ml. Using one with unit-based dosing instructions can cause significant errors.

If you are unsure how many units in insulin syringe you should draw, check with your prescribing pharmacy before injecting.