How much bacteriostatic water should I add to a peptide vial?

There is no single correct volume — it depends on the concentration you want. Adding 2 mL of bacteriostatic water to a 5 mg vial gives 2.5 mg/mL; adding 1 mL to the same vial gives 5 mg/mL. Enter the vial size and your target dose into the calculator and it returns a water volume that lands on a clean syringe mark.

Does adding more bacteriostatic water make a weaker dose?

It makes a weaker concentration, not a weaker dose. The total peptide mass in the vial stays the same regardless of water volume — you just draw a larger syringe volume to reach the same dose. More water means a bigger, easier-to-read draw; less water means a smaller, more concentrated one.

How long does bacteriostatic water last once a vial is opened?

Unopened bacteriostatic water for injection is typically labeled with an expiration date of a few years. Once a peptide vial is reconstituted, it is generally kept refrigerated and used within the timeframe noted on the peptide's own documentation, since the peptide — not the water — is usually the limiting factor.

Can I reuse the same vial of bacteriostatic water for more than one peptide?

Bacteriostatic water's preservative (benzyl alcohol) is what allows a single vial to be drawn from repeatedly without contamination building up, so one vial can supply multiple reconstitutions if sterile technique is used each time. Track how many draws it's had and its own expiration date.

Bacteriostatic Water Calculator: How Much to Add

By PeptCalc Research — Medically reviewed by David Mansour, MD — Last reviewed July 12, 2026

A bacteriostatic water calculator answers one question: how much water turns a vial of lyophilized peptide into a solution you can measure accurately in an insulin syringe. Get the water volume right and every dose after it is arithmetic. Get it wrong and every dose is off by the same margin. This reference walks through the math the calculator runs, so you can check its output or work it by hand. It is written for research and educational purposes only and is not medical advice.

What bacteriostatic water is

Bacteriostatic water (BAC water) is sterile water containing 0.9% benzyl alcohol, a preservative that lets a multi-use vial be drawn from repeatedly without the water itself becoming a contamination risk. It is not the same as sterile water for injection, which carries no preservative and is generally treated as single-use. Peptides ship as freeze-dried powder because the dry form is far more stable than the reconstituted solution — the water is added only when you're ready to start measuring doses. For the full mixing procedure, including how to add the water without damaging the peptide, see how to reconstitute peptides.

The one number the calculator solves for: concentration

Reconstitution comes down to a single ratio:

Concentration = peptide mass ÷ water volume

A 5 mg vial with 2 mL of bacteriostatic water added is 2.5 mg/mL. The same 5 mg vial with 1 mL added is 5 mg/mL — twice as concentrated, because the same mass is dissolved in half the water. Neither number is "correct" on its own. The right volume is whichever one makes your intended dose land on a syringe mark you can actually read.

Vial size	Water added	Concentration
5 mg	2 mL	2.5 mg/mL (2,500 mcg/mL)
5 mg	1 mL	5 mg/mL (5,000 mcg/mL)
10 mg	2 mL	5 mg/mL (5,000 mcg/mL)
10 mg	1 mL	10 mg/mL (10,000 mcg/mL)

From concentration to syringe units

A standard U-100 insulin syringe measures 100 units per mL, so 1 unit equals 0.01 mL. Once you know the concentration, two divisions get you to units:

Concentration (mcg/mL) ÷ 100 = mcg per unit
Target dose (mcg) ÷ mcg per unit = units to draw

Worked example. A 10 mg vial with 2 mL of bacteriostatic water added is 5,000 mcg/mL. Divide by 100: 50 mcg per unit. A 250 mcg dose is 250 ÷ 50 = 5 units. A 500 mcg dose is 10 units.

This is fixed arithmetic — the same two divisions every time — but it's also the step where a dosing error is most likely to happen, not the injection itself. A U-100 syringe's unit scale is constant, so the mg-per-unit result depends entirely on how much water went into the vial. Confuse mg and mcg (1 mg = 1,000 mcg) at this step and every draw after it is off by a factor of 1,000. That's our read on where reconstitution goes wrong most often, and it's the reason a calculator is worth using instead of doing the division by hand under time pressure.

Working backward: picking a water volume for round units

Rather than accepting whatever concentration the vial gives you, a reconstitution calculator can run the math in reverse: tell it your vial size and your usual dose, and it returns the water volume that makes that dose land on a whole or half unit — 5 units instead of 4.7, for example. That's easier to read on a syringe barrel and easier to draw consistently.

Use the peptide reconstitution calculator to enter your vial size and target dose directly — it returns the water volume, concentration, and units to draw without manual arithmetic, plus this reverse mode for hitting a round number. The same math applies whether the vial holds a single peptide such as TB-500 or a multi-peptide blend such as GLOW — only the vial's labeled mass changes.

Storage after reconstitution

Once bacteriostatic water is added, the solution is generally kept refrigerated and shielded from light. The dry powder is stable at room temperature for longer; the reconstituted solution is not, which is why the water is added last. Track the date each vial was mixed rather than relying on memory.

Common mistakes

Adding water without deciding on a target concentration first — the volume should be chosen deliberately, not by habit.
Mixing up mg and mcg, which throws every downstream unit calculation off by 1,000x.
Assuming a 0.5 mL or 0.3 mL "micro" syringe reads the same as a standard U-100 barrel — check what the syringe barrel actually says before drawing.
Shaking instead of swirling the vial after adding water, which can degrade the peptide.
Losing track of which vial was reconstituted with how much water, especially when running more than one peptide at a time.