How to read a U-100 insulin syringe
A U-100 syringe is marked in units, and 100 units is exactly 1 mL. Every number on the barrel is a volume. Once you know what one tick is worth on your barrel, reading it is counting.
What "U-100" means
U-100 is an insulin strength: 100 units of insulin in each millilitre. A U-100 syringe is printed to match, so that the 100 mark sits at 1 mL. That gives one fixed relationship:
For insulin, a unit on the barrel is also a unit of the drug. For anything else drawn into the same syringe, the marks are only a volume scale. How much of a substance sits in "10 units" depends on the concentration of the liquid, which is the subject of units vs mg vs mL.
The three barrel sizes
U-100 syringes are sold in three common capacities. All three use the same scale, 100 units per mL. The difference is how far the scale runs and how finely it is divided.
| Barrel | Scale runs to | Each small tick is usually | Volume per tick |
|---|---|---|---|
| 0.3 mL | 30 units | 1 unit (some are marked in half units) | 0.01 mL (0.005 mL on half-unit barrels) |
| 0.5 mL | 50 units | 1 unit | 0.01 mL |
| 1 mL | 100 units | 2 units | 0.02 mL |
Tick spacing varies between manufacturers, so check your own barrel before trusting the table. The quick test: find two printed numbers next to each other, subtract them, and divide by the number of gaps between them. If the barrel shows 10 and 20 with five gaps between, each tick is 2 units. If there are ten gaps, each tick is 1 unit.
Reading the plunger
The rubber stopper on the end of the plunger has a domed tip and one or two rings. The measurement is read at the top ring, the flat edge of the stopper nearest the needle. The tip of the dome and the lower ring are not the reading line.
Hold the syringe at eye level with the scale facing you. Count up from the nearest printed number below the stopper edge, one tick at a time.
Units to mL at a glance
| Units on a U-100 syringe | Volume |
|---|---|
| 1 unit | 0.01 mL |
| 5 units | 0.05 mL |
| 10 units | 0.1 mL |
| 20 units | 0.2 mL |
| 25 units | 0.25 mL |
| 30 units | 0.3 mL |
| 50 units | 0.5 mL |
| 100 units | 1 mL |
When a number falls between ticks
On a 1 mL barrel with 2-unit ticks, an odd number such as 7 units sits halfway between two lines and has to be judged by eye. On a 0.3 mL or 0.5 mL barrel with 1-unit ticks, 7 units lands on a line. A smaller barrel spreads the same volume over more length, so each unit is easier to see.
If a calculated result is a fraction your barrel cannot show, such as 7.5 units on 1-unit ticks, do not guess. Ask your pharmacist how they want it measured.
What the syringe cannot tell you
- It does not know the concentration. The same 10 units can hold very different amounts depending on how much water went into the vial. The calculator does that step.
- It is not a U-40 or U-500 syringe. Those are printed for other insulin strengths and their marks mean different volumes. See U-100 vs U-40.
- It is not a tuberculin syringe. A 1 mL tuberculin syringe is marked in mL (0.01 mL steps), not units, though 0.10 mL on one equals 10 units on a U-100.
A side-by-side chart of the three barrels is on the syringe units chart page, and the unit converter turns units into mL and back.
Questions
How many mL is 1 unit on an insulin syringe?
On a U-100 syringe 1 unit is 0.01 mL, so 100 units is 1 mL. Divide units by 100 to get mL.
Is a 0.5 mL insulin syringe the same as 50 units?
Yes, on a U-100 syringe. The 0.5 mL barrel is marked from 0 to 50 units, and the 50 mark is 0.5 mL.
Are the units on a syringe the same as mg?
No. Units on a U-100 syringe measure volume. Milligrams measure the amount of a substance. The two are linked only by the concentration of the liquid in the vial, in mg per mL.
Which line of the plunger do I read?
The top ring of the rubber stopper, which is the flat edge closest to the needle. Do not read from the tip of the dome or from the lower ring.