What is a unit on an insulin syringe?
A unit is a volume mark. On a U-100 syringe, 100 units is exactly 1 mL, so one unit is 0.01 mL. It says nothing on its own about how many milligrams you have drawn.
The direct answer
One unit on a U-100 insulin syringe is 0.01 mL. Units measure volume, not mass. Two syringes drawn to the same unit mark hold the same amount of liquid and can still hold very different amounts of peptide.
1 unit = 0.01 mL · 100 units = 1 mL
That is why a mg question cannot be answered from a unit mark alone. 10 mg in 2 mL gives 5 mg/mL, so 10 units on a U-100 syringe contains 0.5 mg. Change the water to 4 mL and the same 10 units contains 0.25 mg.
This page is arithmetic only. It converts numbers you already have from the vial in front of you and the prescription you were given. It does not tell you how much water to add, how much to take, or how often. Those are medical decisions for a licensed clinician who knows your history.
The three common barrels
All three are U-100 scaled. The capacity changes, the meaning of a unit does not.
| Capacity | Holds | One unit | Reading it |
|---|---|---|---|
| 0.3 mL | 30 units | 0.01 mL | The finest scale of the three. Marks are usually printed every single unit, which makes small draws easiest to read. |
| 0.5 mL | 50 units | 0.01 mL | A middle ground. Marks are commonly printed every unit, with numbers every five. |
| 1 mL | 100 units | 0.01 mL | The largest common barrel and the one the calculator draws. Marks are commonly printed every two units, with numbers every ten. |
Units to millilitres
| Units | 5 u | 10 u | 20 u | 25 u | 40 u | 50 u | 75 u | 100 u |
|---|---|---|---|---|---|---|---|---|
| Volume | 0.05 mL | 0.1 mL | 0.2 mL | 0.25 mL | 0.4 mL | 0.5 mL | 0.75 mL | 1 mL |
Ten units on the barrel
Questions people actually type
What does one unit mean on an insulin syringe?
One unit is 0.01 mL of liquid. The U-100 name means the barrel is scaled so 100 units fills exactly 1 mL. The mark measures how much liquid you have drawn, not how much drug is in it, so the same unit mark carries a different mass in every concentration.
Are units and mL the same thing?
They measure the same quantity, volume, on different scales. On a U-100 syringe you convert by dividing units by 100 to get millilitres, or multiplying millilitres by 100 to get units. Fifty units is 0.5 mL, and 0.25 mL is 25 units.
How do 30, 50, and 100 unit syringes differ?
Only in how much they hold and how finely they are printed. All three are U-100 scaled, so a unit is 0.01 mL on every one of them. A 30 unit barrel holds 0.3 mL, a 50 unit barrel holds 0.5 mL, and a 100 unit barrel holds 1 mL. A draw of 20 units is the same volume on all three.
How do you read mL and mg on an insulin syringe?
The barrel only shows volume, so mL is read directly from the scale by dividing units by 100. Milligrams are never printed. To get mass you multiply the volume in mL by the concentration in mg per mL, which comes from the vial mass divided by the water added.
Related conversions
Worked vial cases
Run your own numbers
The free web calculator takes your vial, your water, and your amount, and shows every step of the conversion. Nothing is stored and no account is needed.