BAC Water Calculator — How Much Bacteriostatic Water to Add to Peptides
Two modes: find the exact BAC water volume needed for your target concentration, or calculate the resulting concentration from the amount you added.
Bacteriostatic water (BAC water) is sterile water containing 0.9% benzyl alcohol as a preservative. Unlike plain sterile water — which must be used within 24–48 hours — BAC water inhibits bacterial growth and keeps reconstituted peptide solutions stable in the refrigerator for up to 28 days. It is the standard solvent for reconstituting research peptides including BPC-157, semaglutide, tirzepatide, and retatrutide.
For most 5mg peptide vials: add 2ml of bacteriostatic water → concentration is exactly 2500 mcg/ml. At this concentration, a 250mcg dose is 0.10ml (10 units) on a 100-unit insulin syringe. Use the calculator below for any vial size or target concentration.
Enter your vial size and the concentration you want to achieve. The calculator tells you how much BAC water to add.
Common concentrations:
Formula: ml BAC water = (vial mg × 1,000) ÷ target concentration (mcg/ml)
Enter your vial size and the amount of BAC water you added. The calculator shows the resulting concentration.
Formula: concentration = (vial mg × 1,000) ÷ BAC water (ml)
BAC Water Quick Reference — Common Vial Sizes
Resulting concentrations and per-unit values for the most common vial sizes and BAC water volumes used in peptide research.
| Vial Size | BAC Water Added | Concentration | Per 10 Units (0.1 ml) | Common For |
|---|---|---|---|---|
| 5 mg | 1 ml | 5,000 mcg/ml | 500 mcg | BPC-157 high-dose |
| 5 mg | 2 ml | 2,500 mcg/ml | 250 mcg | BPC-157 standard, TB-500 |
| 5 mg | 5 ml | 1,000 mcg/ml | 100 mcg | Ipamorelin, Sermorelin |
| 10 mg | 2 ml | 5,000 mcg/ml | 500 mcg | Semaglutide, Tirzepatide |
| 10 mg | 4 ml | 2,500 mcg/ml | 250 mcg | Tirzepatide low-dose titration |
| 10 mg | 2 ml | 5,000 mcg/ml | 500 mcg | Retatrutide |
| 2 mg | 1 ml | 2,000 mcg/ml | 200 mcg | Cagrilintide, Tesamorelin |
| 2 mg | 0.5 ml | 4,000 mcg/ml | 400 mcg | Tesamorelin standard |
Choosing the Right Amount of BAC Water
The amount of bacteriostatic water you add to a peptide vial directly determines the concentration of your solution — and that concentration determines how precisely you can measure each dose. There is no single "correct" amount; the right volume depends on your dose, your syringe, and your tolerance for measurement error.
Less BAC water = higher concentration. Higher concentration means each syringe marking represents more peptide. This is efficient for storage (less volume per dose) but requires more precision. A 10-unit difference on an insulin syringe at 5,000 mcg/ml represents 500 mcg — a clinically significant amount for potent peptides.
More BAC water = lower concentration. Lower concentration makes each dose easier to measure accurately. If 250 mcg = 0.25 ml (25 units), a 1-unit measurement error is less significant than if 250 mcg = 0.05 ml (5 units). However, more BAC water means larger injection volumes, which can cause more subcutaneous discomfort.
As a general principle: aim for a concentration that places your dose between 10 and 50 units on a 100-unit insulin syringe. This gives enough resolution to measure accurately without requiring uncomfortably large injection volumes. For most peptides, this means 1–3 ml of BAC water per 5 mg vial, and 2–4 ml per 10 mg vial.
Once reconstituted, store your vial upright in the refrigerator at 2–8°C (36–46°F). Bacteriostatic water preserves the solution for 4–6 weeks. Never freeze a reconstituted peptide — ice crystals destroy the peptide structure. Always use a new needle each time you draw from the vial to maintain sterility.