How much bacteriostatic water do I add to a semaglutide vial?

A common research setup uses 1 mL of bacteriostatic water for every 5 mg of semaglutide, which holds the concentration at 5 mg/mL across vial sizes. A 2 mg vial gets 0.4 mL, a 5 mg vial gets 1 mL, and a 10 mg vial gets 2 mL. Use the calculator to confirm the units for your exact vial and dose.

What concentration should I reconstitute semaglutide to?

5 mg/mL is a common reference concentration for semaglutide research vials because it lands round-number doses on clean unit marks on a U-100 insulin syringe. Scaling the water to the vial size (1 mL per 5 mg) keeps that concentration constant across vial sizes.

How many units is a 1 mg semaglutide dose?

At a 5 mg/mL concentration, a 1 mg dose is 20 units on a U-100 insulin syringe. This holds for any vial size, as long as the water volume was scaled to keep the concentration at 5 mg/mL.

Semaglutide Reconstitution Calculator: Vial & Syringe Math

Q: Is semaglutide bought online actually the purity the label claims?

Not reliably. A peer-reviewed 2024 analysis of semaglutide purchased from online sellers without a prescription found real purity of 7.7%-14.37% against a labeled 99%, with endotoxin detected in every sample tested (JMIR, 2024). Sourcing from a supplier that publishes third-party Certificates of Analysis is the only way to verify what's actually in the vial.

By PeptCalc Research — Medically reviewed by David Mansour, MD — Last reviewed July 10, 2026

Semaglutide is a GLP-1 receptor agonist supplied as lyophilized powder for research use, and it has to be reconstituted with a liquid before it can be measured or drawn into a syringe. This reference explains how a semaglutide reconstitution calculator turns a vial size, water volume, and target dose into the exact number of units to draw on a U-100 insulin syringe. It is written for research purposes only and is not medical advice.

What the calculator does

The peptide calculator handles the reconstitution math in two directions:

Forward mode — enter vial size, water volume, and target dose → it returns the syringe units to draw.
Reverse mode — enter vial size, target dose, and desired syringe units → it tells you how much water to add.

Both modes run on the same formula: concentration equals mass divided by volume, and dose in units equals dose in volume times 100 on a U-100 syringe.

Common semaglutide vial sizes

Research vials are typically supplied in:

2 mg
5 mg
10 mg
15 mg

The calculator supports any vial size. These four are the ones most often listed by research suppliers.

Typical research concentration

A common reconstitution setup for semaglutide scales the water to the vial size at 1 mL per 5 mg, which holds the concentration at a constant 5 mg/mL no matter which vial you're working from:

Vial size	Water added	Concentration
2 mg	0.4 mL	5 mg/mL
5 mg	1 mL	5 mg/mL
10 mg	2 mL	5 mg/mL
15 mg	3 mL	5 mg/mL

Because the ratio is fixed, the units for a given dose don't change with vial size — only the number of full doses the vial holds changes.

Target dose	Units at 5 mg/mL
0.25 mg	5 units
0.5 mg	10 units
1 mg	20 units
2 mg	40 units

A 1 mg dose landing on an exact 20-unit mark is a large part of why 5 mg/mL is a common reference concentration for semaglutide — it reads cleanly on any U-100 syringe.

The math

Step 1: Calculate concentration

Concentration = vial mass ÷ water volume

5 mg vial + 1 mL water = 5 mg/mL
10 mg vial + 2 mL water = 5 mg/mL
10 mg vial + 4 mL water = 2.5 mg/mL

Step 2: Convert dose to volume

Volume (mL) = dose (mg) ÷ concentration (mg/mL)

Example: 1 mg dose at 5 mg/mL concentration = 1 ÷ 5 = 0.2 mL

Step 3: Convert volume to syringe units

On a U-100 insulin syringe, 1 mL = 100 units, so:

Units = volume (mL) × 100

Example: 0.2 mL × 100 = 20 units

The calculator performs these three steps instantly for any vial, water, and dose combination you enter.

Reconstitution process

Gather supplies — the semaglutide vial, bacteriostatic water, U-100 insulin syringes, alcohol swabs.
Swab both vial stoppers with alcohol.
Draw the calculated water volume (e.g. 1 mL for a 5 mg vial at 5 mg/mL).
Inject slowly down the inside wall of the vial, not directly onto the powder.
Swirl gently — do not shake. Let it dissolve completely.
Verify the concentration matches what you entered in the calculator.

For a detailed walkthrough, see how to reconstitute peptides.

Storage

Reconstituted peptide vials are generally stored refrigerated and protected from light. Unreconstituted lyophilized powder is stable at room temperature until the expiration date on the vial. Track the reconstitution date so you know how long a given vial has been mixed.

Why the ratio matters

Some peptides are reconstituted with a fixed water volume regardless of vial size, which means concentration — and therefore units per dose — shifts with vial size. Scaling water to mass instead keeps the concentration, and the units for any given dose, the same across vial sizes:

1 mg dose, any vial size, at the 1 mL per 5 mg ratio:

Vial size	Water added	Concentration	Units for a 1 mg dose
2 mg	0.4 mL	5 mg/mL	20 units
5 mg	1 mL	5 mg/mL	20 units
10 mg	2 mL	5 mg/mL	20 units
15 mg	3 mL	5 mg/mL	20 units

If you'd rather use a different water volume — more water for a bigger, easier-to-read draw, for instance — the semaglutide reconstitution calculator's reverse mode will show the resulting units for any combination.

Sourcing matters as much as the math

Getting the reconstitution math right only matters if the vial actually contains what the label says. Semaglutide is one of the most counterfeited compounds in the unregulated online market, and the data on that is direct: a peer-reviewed 2024 analysis of semaglutide purchased from online sellers without a prescription found real purity of 7.7%-14.37% against a labeled 99%, with endotoxin detected in every sample tested (JMIR, 2024). The same study found actual semaglutide content exceeded the labeled amount by roughly 29%-39% in the samples tested — meaning the concentration on the label didn't match what was actually in the vial (PMC, 2024).

That matters for reconstitution math specifically: every calculation above assumes the vial's labeled mg amount is correct. If the actual content is off from the label, the concentration you calculate is off too, regardless of how precisely you measure water and draw units. A supplier that publishes third-party Certificates of Analysis for each batch is the only way to verify that assumption before it feeds into the math.

Common mistakes

Confusing mg and mcg. Semaglutide doses are small enough that they're sometimes written in micrograms (mcg) instead of milligrams (mg) — 0.25 mg and 250 mcg are the same amount, but reading one as the other is a 1000x error. Double-check your units before drawing.
Shaking the vial. Swirl gently instead — shaking can degrade the peptide.
Not scaling the water to the vial size. Using the same water volume across a 2 mg and a 15 mg vial produces very different concentrations. Recalculate for each vial size rather than reusing a number from memory.
Using a non-U-100 syringe. The calculator assumes a standard U-100 insulin syringe (100 units = 1 mL). If your syringe is marked differently, the unit conversion will not match.

Use the semaglutide reconstitution calculator

Enter your semaglutide vial size, bacteriostatic water volume, and target dose into the free peptide calculator to see the exact syringe units. Reverse mode also works — enter your target dose and the units you want to draw, and it tells you how much water to add.

For related math, see the retatrutide dosage calculator guide, the tirzepatide reconstitution calculator guide, or the general how to reconstitute peptides walkthrough. The calculator handles the math. This guide explains what the numbers mean, and why the source of the vial matters just as much.