A U-100 syringe marks 100 units per mL; a U-40 syringe marks 40 units per mL. So the same 0.1 mL of liquid reads as 10 units on a U-100 barrel and 4 units on a U-40 barrel. A "unit" is a division of the barrel, not a fixed amount of anything. Mixing up the two scales changes the volume by a factor of 2.5, in one direction or the other.
Notice: Research peptides are sold for in-vitro laboratory research only. They are not FDA-approved, not for human or animal consumption, and this page is not medical advice.
This page is about reading syringe scales and converting between them. It does not cover dosing or administration of anything.
Where the two scales come from
Insulin-style syringes are marked in "units" because they were designed to match insulin concentrations. U-100 means 100 units per mL; U-40 means 40 units per mL. U-40 insulin is used in veterinary medicine. Merck Animal Health's Vetsulin, for example, contains 40 International Units per mL, and the company contrasts it with human insulin formulations at 100 IU/mL (Merck Animal Health).
Each syringe scale matches its insulin, so that one printed unit holds one unit of that insulin. Outside of insulin, the unit marks are simply volume marks:
| Scale | Units per mL | Volume per unit | 0.1 mL reads as | Full 1 mL reads as |
|---|---|---|---|---|
| U-100 | 100 | 0.01 mL | 10 units | 100 units |
| U-40 | 40 | 0.025 mL | 4 units | 40 units |
A U-40 unit holds 2.5 times as much liquid as a U-100 unit.
The conversion math
Always convert through volume in mL. That is the one number both scales agree on.
volume (mL) = U-100 units ÷ 100
volume (mL) = U-40 units ÷ 40
U-40 units = U-100 units × 0.4
U-100 units = U-40 units × 2.5
Worked examples:
| U-100 units | Volume | U-40 units |
|---|---|---|
| 5 | 0.05 mL | 2 |
| 10 | 0.10 mL | 4 |
| 20 | 0.20 mL | 8 |
| 25 | 0.25 mL | 10 |
| 50 | 0.50 mL | 20 |
| 100 | 1.00 mL | 40 |
Quick answers: 1 unit on a U-40 syringe is 0.025 mL, the same as 2.5 units on a U-100. 40 units on a U-40 syringe is 1 mL, which is 100 units on a U-100. And 0.1 mL is 10 units on a U-100 but 4 units on a U-40.
Our peptide reconstitution calculator has a U-40 option in the syringe menu. Select it and the unit count is recalculated on a 40-units-per-mL scale, while concentration, draw volume and mcg per 0.1 mL stay the same.
Worked example: one vial, two syringes
Say a 5 mg vial is reconstituted with 2 mL of diluent. The concentration is 5 ÷ 2 = 2.5 mg/mL, or 2,500 mcg/mL. The volume holding 250 mcg is 250 ÷ 2,500 = 0.1 mL.
| Step | U-100 syringe | U-40 syringe |
|---|---|---|
| Concentration | 2,500 mcg/mL | 2,500 mcg/mL |
| Volume for 250 mcg | 0.1 mL | 0.1 mL |
| Units per mL | 100 | 40 |
| Mark on the barrel | 10 units | 4 units |
Everything above the last row is the same. Only the barrel marking changes. That's why the volume, not the unit count, is the number to carry between syringes.
Why mix-ups happen
Most calculators assume U-100
Many online peptide calculators report only U-100 units. If a result says "10 units" and the syringe in hand is U-40, reading to the 10 mark draws 0.25 mL instead of 0.1 mL, 2.5 times the intended volume.
The failure goes both ways
Merck Animal Health warns Vetsulin users that "using a U-100 insulin syringe, would result in a dog receiving two and a half times less insulin than required" (Merck Animal Health). The FDA-approved Vetsulin label puts it in capitals: "USE OF A SYRINGE OTHER THAN A U-40 SYRINGE WILL RESULT IN INCORRECT DOSING" (DailyMed).
Scale confusion is a known error type
Using the wrong syringe scale is a recognized medication-safety problem. The Institute for Safe Medication Practices (ISMP) addresses a related case, U-500 insulin, in its insulin guidelines, stating that patients "should be taught the difference between U-100 and U-500 syringes, and be able to articulate why they should not be used interchangeably" (ISMP). The same logic applies to U-40.
Syringes look alike
U-40 and U-100 syringes come in similar barrel sizes and packaging. The scale is printed on the barrel, but it's easy to miss if you only look at the numbers.
How to read each barrel
U-100 barrels
U-100 syringes come in three common sizes: 0.3 mL (30 units), 0.5 mL (50 units) and 1.0 mL (100 units). On a 1.0 mL barrel, each small tick is usually 2 units. On 0.3 mL and 0.5 mL barrels, ticks are often single units. A 0.1 mL volume is 10 units on every U-100 barrel, whatever its size.
U-40 barrels
A full U-40 barrel holds 1 mL, marked 0 to 40 units. Tick spacing varies by brand, so count the ticks between two labeled numbers before reading. A 0.1 mL volume is 4 units.
Reading technique (both scales)
- Hold the syringe at eye level.
- Read at the leading edge of the rubber plunger stopper, the flat ring touching the liquid.
- Tap out and expel air bubbles before reading. A bubble takes up space, so the barrel holds less liquid than the mark shows.
Five checks before you trust a unit number
- Read the scale on the barrel. Look for "U-100" or "U-40" printed on it.
- Read the scale on the box. Confirm it matches.
- Match the calculator setting. Select U-40 in the calculator if that's what you have.
- Record volume, not units. Write mL or mcg per 0.1 mL in your notes. Units only make sense with a scale attached.
- Don't mix boxes. If you have both types, store them apart.
Why mcg per 0.1 mL is the safer number
Syringe units describe the syringe. Concentration describes the solution. A vial at 2,500 mcg/mL holds 250 mcg in every 0.1 mL whether you measure with a U-100 barrel, a U-40 barrel or a pipette. That's why our calculator reports mcg per 0.1 mL next to syringe units. If you write down only one number, make it that one.
For the full reconstitution procedure, see how to reconstitute peptides. For mg, mcg and IU conversions, see our conversion guide and unit converter.
Accurate math starts with an accurate label
Unit conversion assumes the vial holds what its label says. Check that the vendor publishes lot-matched third-party testing before relying on the label (how to read a peptide COA). Our ranking of the best place to buy peptides online scores US vendors on testing and labeling. It is an editors' ranking, not a sort by score (how we rank).
Sources are linked inline. Last reviewed September 26, 2026.