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

It depends on the concentration you want. A common research setup is 2 mL of bacteriostatic water per 5 mg vial, giving 2.5 mg/mL. Use the calculator to pick a water volume that lands your dose on a round syringe mark.

What is the formula for peptide concentration?

Concentration equals the peptide mass in the vial divided by the volume of bacteriostatic water added. For example, 10 mg in 2 mL equals 5 mg/mL, or 5000 mcg/mL.

How do I convert a peptide dose to insulin-syringe units?

On a U-100 syringe, 1 mL equals 100 units. Divide the concentration in mcg/mL by 100 to get mcg per unit, then divide your target dose by that number to get the units to draw.

How to Reconstitute Peptides: Step-by-Step With Syringe Math

By PeptCalc Research — Medically reviewed by David Mansour, MD — Last reviewed July 8, 2026

Peptides are typically supplied as lyophilized (freeze-dried) powder that must be reconstituted with a liquid before it can be measured. This reference walks through the reconstitution process and the arithmetic that turns a target dose into an exact number of units on an insulin syringe. It is written for research and educational purposes only and is not medical advice.

What you need

The peptide vial (labeled in milligrams, e.g. 5 mg or 10 mg)
Bacteriostatic water (BAC water) — sterile water containing 0.9% benzyl alcohol, which lets a multi-use vial be drawn from repeatedly
U-100 insulin syringes
Alcohol swabs

Step 1 — Add the bacteriostatic water

Swab both vial stoppers. Draw your chosen volume of BAC water into a syringe and inject it slowly down the inside wall of the peptide vial rather than directly onto the powder. Do not shake — gently swirl or let it dissolve on its own. The volume of water you add sets the concentration, so choose it deliberately (see the calculator).

Step 2 — Work out the concentration

Concentration is simply mass ÷ volume:

5 mg in 2 mL = 2.5 mg/mL = 2500 mcg/mL
10 mg in 2 mL = 5 mg/mL = 5000 mcg/mL
10 mg in 1 mL = 10 mg/mL = 10000 mcg/mL

More water means a lower concentration and a larger, easier-to-read draw. Less water means a higher concentration and a smaller draw.

Step 3 — Convert the dose to syringe units

A U-100 insulin syringe holds 100 units per mL, so 1 unit = 0.01 mL. To find units per dose:

Divide concentration (mcg/mL) by 100 → mcg per unit
Divide your target dose (mcg) by mcg per unit → units to draw

Worked example. 10 mg vial + 2 mL water = 5000 mcg/mL. That is 5000 ÷ 100 = 50 mcg per unit. A 250 mcg dose is 250 ÷ 50 = 5 units. A 500 mcg dose is 10 units.

Rather than doing this by hand each time, enter your numbers into the peptide reconstitution calculator and it returns the exact units, plus a reverse mode that tells you how much water to add to hit a round unit mark.

Bacteriostatic vs. sterile water

Both are sterile, but only bacteriostatic water contains the preservative that inhibits microbial growth across repeated needle entries. Sterile water for injection has no preservative and is generally treated as single-use. Storage windows differ, so track the date a vial was reconstituted.

Storage

Reconstituted peptides are generally kept refrigerated and protected from light. Unmixed lyophilized powder is far more stable than the reconstituted solution, which is why vials are shipped dry.

Common mistakes

Shaking the vial instead of swirling
Spraying water directly on the powder at high pressure
Mixing up mg and mcg (1 mg = 1000 mcg) — the single most common source of a wrong draw
Forgetting that syringe units are read on a U-100 scale unless the barrel says otherwise