Dosing reference
Insulin Syringe Units Explained: U-100 vs U-40
By Baris Bingor · Last updated
What does a unit mean on an insulin syringe?
A 'unit' on an insulin syringe is a volume mark, not a fixed amount of drug. On a U-100 syringe — the standard for peptides — 100 units = 1 ml, so 1 unit = 0.01 ml and 10 units = 0.1 ml. On a U-40 syringe, 40 units = 1 ml (1 unit = 0.025 ml). To get the mass per unit, multiply the volume by your solution's concentration in mg/ml.
What a "unit" actually is
The numbers printed on an insulin syringe barrel are units — and a unit is just a volume graduation, not an amount of drug. The whole scale is calibrated to a specific insulin concentration. On the standard U-100 syringe, the barrel is divided so that 100 units fill exactly 1 ml. That makes one unit equal to 0.01 ml, ten units 0.1 ml, and fifty units half a millilitre. Nothing about the unit mark knows what is inside the syringe — it is a fine volume ruler, which is exactly why it is so useful for the small liquid volumes in peptide reconstitution.
U-100 vs U-40: the only real difference
"U-100" and "U-40" describe how many units of liquid fit in one millilitre — the calibration of the scale. A U-100 syringe packs 100 units into 1 ml; a U-40 syringe packs only 40. So U-100 marks are finer (each unit is a smaller slice of liquid) and U-40 marks are coarser. The practical consequence: the same unit count is a different volume on each syringe.
Units → millilitres on each syringe
| Units drawn | Volume on U-100 | Volume on U-40 |
|---|---|---|
| 1 unit | 0.01 ml | 0.025 ml |
| 10 units | 0.10 ml | 0.25 ml |
| 20 units | 0.20 ml | 0.50 ml |
| 40 units | 0.40 ml | 1.00 ml |
| 100 units | 1.00 ml | 2.50 ml |
Read the table across: 10 units is 0.1 ml on a U-100 syringe but 0.25 ml on a U-40 — 2.5× the liquid. That 2.5× factor is exactly the ratio between the two calibrations (100 ÷ 40).
Why peptide dosing uses U-100
Almost every peptide reconstitution protocol expresses the draw in U-100 units, for two reasons. First, U-100 insulin syringes are by far the most widely available. Second, the fine 0.01 ml graduation is ideal for the small volumes peptides are drawn in — a 0.1–0.5 ml draw lands at a clean, readable 10–50 units. When a protocol says "draw 20 units," it means 0.2 ml on a U-100 syringe.
Units → milligrams (the part people get wrong)
There is no universal units-to-milligrams conversion for peptides, because units measure volume and milligrams measure mass. The bridge between them is your solution's concentration:
- Volume = units × 0.01 ml (on a U-100 syringe).
- Mass = volume (ml) × concentration (mg/ml).
For example, 10 units of a 5 mg/ml solution is 0.1 ml × 5 mg/ml = 0.5 mg. Change the concentration and the same 10 units delivers a different mass. A common error is to apply insulin's "1 IU ≈ 0.0347 mg" figure to a peptide — that number is specific to insulin's biological potency and is meaningless for peptides, where the syringe unit is nothing more than a volume mark.
Reading half-units and small draws
Many doses do not land on a whole unit — a 12.5-unit draw sits between the 12 and 13 marks. On a paper scale that is a squint; in Peptly the visual syringe view shows the exact fill point, so a half-unit draw is unambiguous.
Common mistakes
- Mixing up U-100 and U-40. Using the wrong syringe for your computed unit count over- or under-doses by 2.5×.
- Treating units as milligrams. Units are volume; mass depends on concentration.
- Applying an insulin IU→mg number to a peptide. It does not transfer.
- Confusing units with "clicks" on a pen. Pen dose dials are a separate system from syringe unit marks.
How Peptly fits in
Peptly works in U-100 units by default (U-40 is selectable in Settings). Set your vial mass, water volume, and target dose, and the app converts dose ↔ volume ↔ units live, then draws the exact mark on a visual syringe — so you never have to do the 0.01 ml arithmetic by hand or count barrel lines under bad lighting.
See also
- Syringe sizes & needle gauge
- Peptide dosage unit reference
- BAC water calculator — pick the right mix volume
- How to reconstitute peptides — step-by-step
- Peptide reconstitution math reference
Frequently asked questions
Is a unit the same as a milliliter? +
No. A unit is a volume mark on the syringe barrel, not a milliliter. On a U-100 syringe, 1 unit = 0.01 ml (100 units = 1 ml). On a U-40 syringe, 1 unit = 0.025 ml (40 units = 1 ml). The unit mark always means a fixed volume for that syringe — it does not measure the drug mass directly.
How many units is 0.5 ml on a U-100 syringe? +
Fifty units. On a U-100 syringe you multiply millilitres by 100, so 0.5 ml = 50 units. On a U-40 syringe the same 0.5 ml would read as 20 units (0.5 × 40). Same liquid volume, different number on the barrel.
How do I convert insulin units to mg for a peptide? +
There is no fixed units-to-mg number. Compute it: mg = units × 0.01 ml (on U-100) × your solution concentration in mg/ml. Example: 10 units of a 5 mg/ml solution = 0.1 ml × 5 mg/ml = 0.5 mg. The insulin "1 IU ≈ 0.0347 mg" figure is specific to insulin potency and does not apply to peptides — for a peptide the syringe unit is purely a volume mark.
Can I use a U-40 syringe for a peptide dose measured in U-100 units? +
Not interchangeably. The same unit count is a different volume on each syringe: 10 units is 0.1 ml on U-100 but 0.25 ml on U-40 — 2.5 times more liquid. Always match the syringe to the scale your dose was computed in. Peptide protocols are virtually always expressed in U-100 units.
Why does Peptly default to U-100? +
U-100 insulin syringes are the most common and the scale peptide protocols specify. Peptly supports U-40 in Settings if you need it; either way the visual syringe view shows the exact draw mark so you are not counting tiny lines by eye.