Concentration (mcg/mL)

Also known as: concentration · peptide concentration

Concentration (mcg/mL) is how many micrograms of peptide are in each milliliter of solution; it converts a dose by weight into a volume to draw.

After reconstitution, the peptide is spread evenly through the liquid in the vial. Concentration describes that: the mass of peptide per milliliter. It is set by two numbers, the peptide mass in the vial and the volume of liquid you added. One milligram is 1000 micrograms, so a vial's mass in mg times 1000 gives its mass in mcg.

Concentration is what turns a dose written in micrograms into a volume, and on a U-100 insulin syringe, into units. More water gives a lower concentration, so each dose takes up more volume; less water gives a higher concentration, so each dose takes less volume but small measuring errors matter more.

Why it matters

Every dose you draw depends on this number being right. If you calculate it with the wrong water volume, or someone else's water volume, every dose from that vial will be wrong by the same proportion.

The number also assumes the labeled mass is accurate. A test-purchase study of semaglutide vials from illegal online pharmacies found content well above the label, which would make the real concentration different from the calculated one. A batch-specific COA reduces that uncertainty but does not remove it. DoseCraft's reconstitution calculator (/tools/reconstitution) runs these formulas for you.

How to calculate it

Convert the vial mass to micrograms: mg × 1000.
Divide by the milliliters of water added to get mcg/mL.
To find the volume for a dose: dose mcg ÷ concentration mcg/mL = mL.
To find U-100 syringe units: mL × 100.
If the answer is not a whole number of units, consider a water volume that gives one. For example, 250 mcg at 3000 mcg/mL is 250 ÷ 3000 × 100 = 8.33 units, which is hard to measure, while at 2500 mcg/mL it is exactly 10 units.
Write the concentration on the vial label and in your log.

Concentration (mcg/mL) = vial mg × 1000 ÷ mL of water; units to draw = dose mcg ÷ concentration mcg/mL × 100

Worked example

A 10 mg vial with 2 mL of water: 10 × 1000 ÷ 2 = 5000 mcg/mL. A 500 mcg dose = 500 ÷ 5000 = 0.1 mL = 10 units. With 4 mL of water instead: 10 × 1000 ÷ 4 = 2500 mcg/mL, and the same 500 mcg dose = 500 ÷ 2500 × 100 = 20 units. Check: 0.2 mL × 2500 = 500 mcg.

Common mistakes

Forgetting to convert mg to mcg, which makes the result 1000 times too small.
Using a different water volume from the one actually added.
Copying a unit number from someone using a different concentration.
Not labeling the vial with its concentration.
Assuming the labeled vial mass is exact.

Sources

Prescribing informationEGRIFTA WR (tesamorelin) for injection - prescribing information and instructions for use. DailyMed, U.S. National Library of Medicine, 2025.
Prescribing informationHumulin R U-100 (insulin human) injection - prescribing information. DailyMed, U.S. National Library of Medicine, 2023.
ReferenceMultifactor Quality and Safety Analysis of Semaglutide Products Sold by Online Sellers Without a Prescription: Market Surveillance, Content Analysis, and Product Purchase Evaluation Study. J Med Internet Res, 2024. PMID 39509151.

Page updated October 3, 2026. How we grade evidence: methodology.

Related Terms

Reconstitution

Reconstitution is dissolving a freeze-dried (lyophilized) powder in a measured volume of liquid to make a solution of known concentration for injection.

Units (Insulin Syringe)

Units on an insulin syringe are volume markings; on a U-100 syringe, 1 unit = 0.01 mL and 100 units = 1 mL. They are not a dose by themselves.

Drawing Dose

Drawing a dose means pulling a measured volume of solution from a vial into a syringe, removing air bubbles, and setting it to the exact mark.

BAC Water

Bacteriostatic water (BAC water) is sterile water for injection with 0.9% benzyl alcohol added as a preservative, used to dissolve drugs for injection.

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