Peptide Vial Math

How to reconstitute Semaglutide

Add a measured volume of bacteriostatic water to the Semaglutide vial and swirl gently. 5 mg + 2 mL = 2.5 mg/mL, so 0.25 mg is 0.1 mL, or 10 units on a U-100 syringe.

Semaglutide vials come in 2, 5, 10, 20 mg, so what a unit holds depends on your vial.

By Peptide Vial Math Editorial Team Reviewed by the team on (how we review) Updated Formulas unit-tested (methodology)

Semaglutide reconstitution steps

  1. Read the vial label and confirm the total mg. This page covers 2, 5, 10, 20 mg.
  2. Choose the water volume using the matrix below and write down the resulting mg/mL and mg per unit.
  3. Warm the closed vial to room temperature; swab both stoppers with alcohol and let them dry.
  4. Add the bacteriostatic water slowly down the inside wall of the vial.
  5. Swirl gently until dissolved; don't shake. Inspect: clear, no particles.
  6. Label with total mg, mL added, mg/mL, mg per unit and the date.

The full procedure, with reasons for each step, is in the general reconstitution guide.

What concentration does each Semaglutide vial and water volume give?

What one U-100 unit holds for each Semaglutide vial size and water volume:

Semaglutide: mcg per U-100 unit (mg/mL in brackets) by vial size and water added
Vial1 mL2 mL2.5 mL3 mL
2 mg 20 (2)10 (1)8 (0.8)6.67 (0.667)
5 mg 50 (5)25 (2.5)20 (2)16.67 (1.67)
10 mg 100 (10)50 (5)40 (4)33.33 (3.33)
20 mg 200 (20)100 (10)80 (8)66.67 (6.67)

And the concentration each combination gives:

Semaglutide: concentration, mg/mL by vial size and water added
Vial1 mL2 mL2.5 mL3 mL
2 mg 210.80.667
5 mg 52.521.67
10 mg 10543.33
20 mg 201086.67

Semaglutide amounts are small (fractions of a milligram), so the reading on the syringe is small too and easy to misread by a factor of ten. FDA reports describe people drawing 50 units instead of 5 units from multi-dose vials. Check that the mL figure and the units figure agree before relying on either.

Semaglutide reconstitution calculator

Vial mg, water mL and amount give syringe units

mg

Total printed on the vial label.

mL

Diluent volume you add (1 mL = 100 units on a U-100 syringe).

Amount unit

The mass you want in one draw. 1 mg = 1,000 mcg.

Syringe barrel
Show U-40 and U-50 syringes

Result

10units

to draw on a U-100 syringe, 100-unit barrel

Vial: 5 mg with 2 mL diluent 5 mg +2 mL
Syringe filled to 10.0 of 100 units 0 10 20 30 40 50 60 70 80 90 100 U-100, 1 mL
Volume
0.1 mL
Concentration
2.5 mg/mL
Per unit
25 mcg
Draws per vial
20

5 mg + 2 mL = 2.5 mg/mL, so 250 mcg is 0.1 mL, or 10 units on a U-100 syringe.

Arithmetic only. Not a dosing recommendation. Check every figure against your vial label and lab protocol.

Per-amount unit charts for each vial size: Semaglutide calculator (2 mg, 5 mg, 10 mg, 20 mg).

Frequently asked questions

How much bacteriostatic water do you add to a 5 mg Semaglutide vial?

There is no single correct volume; it sets the concentration. 5 mg in 1 mL is 5 mg/mL, in 2 mL is 2.5 mg/mL, in 3 mL is 1.67 mg/mL. With 2 mL, each U-100 unit holds 0.025 mg.

How do I reconstitute a 20 mg Semaglutide vial?

Same procedure, different numbers. 20 mg in 2 mL is 10 mg/mL, so each unit holds 0.1 mg, 4× more than a 5 mg vial with the same water. Units copied from a smaller vial will be wrong.

What is the Semaglutide reconstitution formula?

mg/mL = vial mg ÷ water mL; mL = amount ÷ mg/mL; U-100 units = mL × 100. Example: 5 mg + 2 mL = 2.5 mg/mL, so 0.25 mg is 0.1 mL, or 10 units on a U-100 syringe.

Can I change the water volume after reconstituting Semaglutide?

Adding more diluent later is possible arithmetically (the new concentration is the remaining mg ÷ the new total volume), but you must account for what has already been drawn. It is simpler and less error-prone to choose the volume once, before mixing, and label it.

How should the Semaglutide vial be stored after mixing?

Reconstitution changes how a vial has to be kept: a lyophilized powder and a solution have different temperature, light and time limits, and an opened multi-dose diluent vial has its own clock (CDC guidance: discard 28 days after first puncture unless the manufacturer gives another date). The peptide storage guide covers both states, and the bacteriostatic water guide covers the diluent. Label each vial with the amount, the water added, the concentration and the date.

Sources

  1. Wegovy (semaglutide) injection: full prescribing information (revised 06/2026) — Novo Nordisk (FDA-approved label), 2026
  2. FDA alerts health care providers, compounders and patients of dosing errors associated with compounded injectable semaglutide products — U.S. Food and Drug Administration, 2024
  3. FDA's Concerns with Unapproved GLP-1 Drugs Used for Weight Loss — U.S. Food and Drug Administration
  4. Once-Weekly Semaglutide in Adults with Overweight or Obesity (STEP 1) — N Engl J Med, 2021, PMID 33567185
  5. Handling and Storage Guidelines for Peptides — Bachem Knowledge Center
  6. Bacteriostatic Water for Injection, USP (Hospira) — prescribing label — DailyMed, U.S. National Library of Medicine

Links checked 2026-10-19.