What bacteriostatic water is
Bacteriostatic water, usually shortened to BAC water, is sterile water containing a small amount of benzyl alcohol, typically 0.9%, as a preservative. In reconstitution it is the diluent: the liquid added to a freeze-dried powder to make a solution.
For the arithmetic, only one property of the water matters, and that is its volume. The volume you add sets the concentration of the vial, and the concentration decides where any given dose falls on a syringe.
How the BAC water calculator works
Most calculators take the water volume as an input. This one solves for it. You choose the syringe mark, and it finds the volume that puts your dose exactly there.
The target volume is the syringe mark divided by 100, because a U-100 syringe has 100 units per mL. A dose entered in mcg is divided by 1,000 first to convert it to mg.
- Enter the vial size in mg, as printed on the label.
- Enter the dose to measure, from your own records or prescription.
- Pick a syringe mark that is easy to read, such as 10 or 20 units.
- Read the water volume, then check the comparison table to see how other volumes would read.
Peptide dilution: what changes and what does not
Peptide dilution is often misunderstood. Adding more bacteriostatic water does not change how much peptide is in the vial. It spreads the same amount through more liquid.
- What changes: the concentration, the amount held by each unit mark, and how far up the syringe a given dose reads.
- What stays the same: the total in the vial and the number of doses it contains.
That is why the calculator’s “Doses per vial” figure is labelled “same at any water volume”. The water only decides how convenient the reading is.
Worked example of the dilution math
These numbers are arbitrary and chosen because they divide cleanly. They illustrate the formula and are not a suggested amount.
- Vial size: 5 mg. Dose entered: 250 mcg, which is 0.25 mg.
- Target mark: 10 units, which is 10 ÷ 100 = 0.1 mL.
- Water: 5 × 0.1 ÷ 0.25 = 2 mL.
- Check: 5 mg ÷ 2 mL = 2.5 mg/mL, and 0.25 mg ÷ 2.5 mg/mL = 0.1 mL = 10 units.
| Water added | Concentration | 250 mcg reads as | On the scale |
|---|---|---|---|
| 1 mL | 5 mg/mL | 5 units | Whole unit mark |
| 1.5 mL | 3.33 mg/mL | 7.5 units | Between marks |
| 2 mL | 2.5 mg/mL | 10 units | Whole unit mark |
| 3 mL | 1.67 mg/mL | 15 units | Whole unit mark |
Choosing an easy-to-read syringe mark
Insulin syringes are printed with a line at every unit on most 0.3 mL and 0.5 mL barrels, and at every 2 units on most 1 mL barrels. A reading that falls on a printed line is easier to see than one that sits between two lines.
- Very small readings, under about 2 units, are hard to see on any barrel.
- A smaller barrel spreads the same marks further apart, which helps with small volumes.
- The target mark has to exist on your syringe. There is no 60-unit mark on a 0.5 mL barrel.
Syringe scales differ, so see the guide to U-100, U-50 and U-40 syringes if you are unsure which one you have.
Limits of the math
- Instructions come first. If the manufacturer, your pharmacy or your healthcare provider gives a water volume, use it. Then run the numbers in the peptide calculator.
- The math cannot see your vial. A result may be more water than the vial physically holds. Choose a lower mark if so.
- It does not recommend, suggest or verify any dose, and it does not recommend a water volume. It shows the arithmetic for the mark you chose.
- It does not explain how to handle, mix or store a product, and it is not medical advice.
Results are for research, informational and record-keeping reference only. Read the full disclaimer, check a concentration with the reconstitution calculator, or head back to the Peptrack homepage. The free Peptrack app keeps the volume and concentration saved with each vial.
