How much bacteriostatic water should I use to reconstitute a peptide?

The amount depends on vial size and desired concentration. For a standard 5mg vial, adding 2mL of bacteriostatic water creates a concentration of 2.5mg per 1mL. Every 10 units (0.1mL) on an insulin syringe equals 250mcg.

How do you read a 1mL insulin syringe?

A standard 1mL insulin syringe is divided into 100 units. 10 units equal 0.1mL, 50 units equal 0.5mL, and 100 units equal a full 1.0mL.

Can I use sterile water instead of bacteriostatic water?

No, not if you plan to use the vial for multiple doses. Plain sterile water does not contain the benzyl alcohol preservative. A vial mixed with plain sterile water must be used immediately because it will begin growing bacteria within 24 hours.

How long do reconstituted peptides last?

When mixed with bacteriostatic water and stored in the refrigerator, most research peptides maintain potency and sterility for 28 to 30 days.

Peptide Dosage Calculator: Reconstitution and Syringe Math Made Easy

Use our interactive Peptide Dosage Calculator to skip the math.

If you are using compounded or research peptides, you are likely receiving them in a lyophilized (freeze-dried) powder form. Before you can use them, you must "reconstitute" them by mixing the powder with bacteriostatic water.

This is where the math gets confusing. You have a vial measured in milligrams (mg), a dose measured in micrograms (mcg), and a syringe measured in units (U) or milliliters (mL).

This guide breaks down exactly how to perform the math yourself and how to safely reconstitute your peptides.

The Three Variables You Need to Know

To calculate your dose accurately, you only need three numbers:

Vial Size (in mg): How much peptide powder is in the vial? (e.g., 5mg, 10mg)
Water Added (in mL): How much bacteriostatic water did you add to the vial? (e.g., 1mL, 2mL, 3mL)
Desired Dose (in mcg or mg): What is your target dose? (e.g., 250mcg, 500mcg, 2.5mg)

Key conversion: 1 milligram (mg) = 1,000 micrograms (mcg).

Step 1: Reconstitution (Adding the Water)

Bacteriostatic water (BAC water) is sterile water containing 0.9% benzyl alcohol, which prevents bacterial growth and extends the shelf life of the peptide once mixed.

How much water should you add? The amount of water does not change the total amount of peptide in the vial. It only changes the concentration. Adding more water dilutes the mixture, meaning you need to draw more liquid for the same dose. Adding less water makes it concentrated, meaning a tiny error on the syringe could result in a significant overdose.

The Standard 2mL Rule: For most 5mg or 10mg vials, adding exactly 2mL of bacteriostatic water creates a concentration that makes the syringe math simple.

Step 2: The Syringe Math

Insulin syringes are the standard for subcutaneous peptide injections. They are typically 1mL in volume and marked with 100 "units."

100 units = 1.0 mL 10 units = 0.1 mL

Example Calculation (BPC-157)

You have a 5mg vial of BPC-157, and your target dose is 250mcg.

Add 2mL of BAC water to the 5mg vial.
Convert the vial to mcg: 5mg = 5,000mcg.
Find the concentration: 5,000mcg / 2mL = 2,500mcg per 1mL.
Find the unit value: 2,500mcg / 100 units = 25mcg per unit.
Calculate the draw: 250mcg / 25mcg per unit = 10 units.
The Result: Draw the syringe to the 10 unit mark (0.1mL).

Don't want to do the math? Use our Peptide Dosage Calculator or Reconstitution Calculator.

Best Practices for Reconstitution

Never shake the vial. Peptides are fragile amino acid chains. Vigorously shaking can damage the molecular structure. Gently roll the vial between your palms or let it sit until dissolved.
Inject the water slowly. Angle the needle so the water runs down the inside glass wall of the vial, rather than spraying directly onto the powder.
Refrigerate immediately. Once reconstituted, store at 2-8°C (36-46°F). Most peptides remain stable for 28 to 30 days after mixing.
Keep it dark. Peptides are sensitive to UV light. Store in a dark box or the back of the refrigerator.