BAC Water Calculator

Bacteriostatic water is sterile water for injection that contains 0.9% benzyl alcohol as a preservative. The benzyl alcohol inhibits the growth of bacteria and other microorganisms, making the solution safe for repeated use over multiple draws — typically up to 28 days after first puncture when stored properly.

BAC water is different from sterile water for injection (SWFI), which contains no preservative and is intended for single-use only. It is also different from normal saline (0.9% NaCl), which contains sodium chloride. For most peptide reconstitutions, bacteriostatic water is the preferred choice because it allows the vial to be used over multiple days without risking bacterial contamination.

The concentration you target depends on two practical factors: dose accuracy and injection comfort.

• Higher concentration (less water): Produces smaller injection volumes, which can be more comfortable. However, very small draw volumes (under 3-5 units) become harder to measure precisely on an insulin syringe.
• Lower concentration (more water): Produces larger draw volumes that are easier to measure accurately. The tradeoff is a slightly larger injection volume, which is rarely an issue for volumes under 0.5 mL.

A practical rule of thumb: aim for a per-dose draw volume between 5 and 20 units on a U-100 insulin syringe. This keeps the injection small while maintaining good measurement precision. If you are dosing a very small amount (under 100 mcg), using more water gives you a more dilute solution that is easier to measure.

Unopened bacteriostatic water has a long shelf life and should be stored at room temperature (15-30°C / 59-86°F) away from direct light. Once the vial is punctured, it should be used within 28 days — this is the standard beyond-use date for multi-dose vials per USP guidelines.

Reconstituted peptide solutions (peptide + BAC water) should be stored refrigerated at 2-8°C (36-46°F) and also used within 28-30 days. Never freeze a reconstituted peptide solution, as freezing can cause the peptide to aggregate and lose potency. For more storage details, see our How to Store Peptides guide.

For a complete reconstitution walkthrough including sterile technique, see How to Reconstitute Peptides. To convert your concentration into syringe units for a specific dose, use the Peptide Reconstitution Calculator. For compound-specific dosage information, check the Dosage Calculator.