Every calculation on this site ends in the same place: a number of units on an insulin syringe. If you cannot read that number correctly, the rest of the maths does not matter. This guide explains what a unit actually is, why the same number means the same volume on every syringe, and how to read the marks without guessing.
Try the calculator with your own vial size and water volume.
What “U-100” means.
Peptide doses are measured with insulin syringes, and almost all of them are marked U-100. The “U” stands for units, and 100 means the scale is calibrated so that 100 units = 1 mL. One unit is therefore exactly 0.01 mL — every time, on every U-100 syringe. The FDA’s Humulin R label puts it plainly: U-100 insulin contains 100 units/mL, and doses of insulin are measured in units.
Here is the distinction that matters most: on a U-100 syringe used for a peptide solution, the unit markings are a volume scale, not a mass or dose scale. One unit is 0.01 mL of liquid, whatever is in the syringe. How much peptide (in mg or mcg) is in that volume depends entirely on the concentration of the solution in the vial. Units do not equal mcg, and units do not equal mg. (Insulin “units” historically reflect biological activity rather than weight — but for reading a U-100 syringe as a volume scale, the conversion is purely 100 units = 1 mL.)
The conversion.
Units = mL × 100. mL = units ÷ 100. 1 unit = 0.01 mL = 10 microlitres.
| Units (U-100) | Volume | Units (U-100) | Volume |
|---|---|---|---|
| 1 unit | 0.01 mL | 25 units | 0.25 mL |
| 5 units | 0.05 mL | 30 units | 0.30 mL |
| 10 units | 0.10 mL | 50 units | 0.50 mL |
| 15 units | 0.15 mL | 75 units | 0.75 mL |
| 20 units | 0.20 mL | 100 units | 1.00 mL |
Worked volume examples: 17 units = 0.17 mL; 8 units = 0.08 mL; 0.1 mL = 10 units; 0.25 mL = 25 units; 0.5 mL = 50 units.
The three barrel sizes.
The barrel size changes only the syringe’s capacity and how widely spaced the marks are — never the maths. A dose of 10 units is 0.1 mL on all three.
| Barrel | Capacity | Typical marks |
|---|---|---|
| 0.3 mL | 30 units | Every 1 unit (half-unit models exist — see below) |
| 0.5 mL | 50 units | Every 1 unit |
| 1 mL | 100 units | Every 2 units |
Tick spacing — check your own barrel.
Marking steps vary by brand and model, so count the ticks on your own syringe rather than assuming. The usual pattern: on a 1 mL barrel the numbered lines are every 10 units and the small ticks between them are 2 units each — so a 1 mL barrel with 2-unit steps cannot show an odd count such as 15 units exactly. On a 0.5 mL barrel the small ticks are usually 1 unit each. On a 0.3 mL barrel, sources disagree: most say 1 unit per tick, but half-unit (0.5-unit) models exist, and one source reports 0.5 units per tick with 60 ticks total. So for a 0.3 mL barrel, the honest rule is: usually 1 unit per tick, but half-unit models exist — count the ticks on your own barrel before relying on them. Smaller barrels spread the same dose over wider, easier-to-read marks, which is why they are recommended for small volumes; one source advises avoiding the 1 mL barrel for draws under 20 units (source guidance, not a measured standard).
Reading the marks accurately.
Read at the top flat edge of the rubber stopper — the edge nearest the needle, the leading edge the liquid sits against. Do not read from the bottom ring, the far edge, or the domed centre of the stopper; reading the wrong edge is reported to introduce a 1–2 unit error on every draw. Hold the syringe at eye level, needle up, and look at it square-on: reading from above or below creates parallax error, and on a 0.3 mL barrel that shift can be worth a whole unit. Zero is at the needle end; the scale runs from 0 to the maximum capacity (30, 50 or 100 units). And clear the air first: bubbles displace liquid and make the reading inaccurate, so hold the syringe needle-up, tap the barrel gently to move bubbles to the top, push the plunger slightly to expel the air, then re-draw to the correct line and read again.
Warning: U-40 syringes are different.
Veterinary insulin syringes are often U-40: 40 units = 1 mL. Every calculation on this site assumes U-100. Drawing a U-100 calculation on a U-40 syringe delivers two and a half times the intended volume. Check the marking printed on your syringe barrel before you rely on any unit figure.
FAQs
Is 10 units the same as 10 mL?
No — and this confusion is dangerous. On a U-100 syringe, 10 units is 0.1 mL. The numbers on the barrel are units, not millilitres.
Does a bigger syringe change my dose?
No. 20 units is 0.2 mL on a 0.3 mL, 0.5 mL or 1 mL U-100 syringe alike. Only the capacity and the spacing of the marks change.
Why does my dose land between two marks?
Because your concentration and dose produce a number your barrel does not print — for example 15 units on a syringe marked every 2 units. A smaller barrel with finer marks, or a different water volume (confirmed with your provider), can make the same dose land on a printed mark.
Can I use a non-insulin syringe?
Standard syringes are marked in mL only, so you would work in mL (dose mL = units ÷ 100) rather than units. The maths still holds, but the fine marks of an insulin syringe make small volumes far easier to measure. When in doubt, ask a pharmacist which syringe matches your prescription.
What is a “unit” not?
A unit on a U-100 syringe is not 1 mg, not 1 mcg, and not 1 IU by itself. It is a volume mark: 1 unit = 0.01 mL on a U-100 barrel. Converting units to mg or mcg requires the concentration of the solution — a separate step the calculator performs.
Related guides
See also the Reconstitution Guide, the mg to mcg Conversion Guide and the Peptide Calculator home page.
Educational reference only — not medical advice. This guide describes maths and commonly reported measurement practice only; it does not recommend any dose. Always confirm your dose and preparation with a licensed healthcare provider or pharmacist.