Skip to main content
MyPeptideMatch logoMyPeptideMatch

BAC Water Calculator for Peptides

Free BAC water calculator — answers how much bacteriostatic water should I add? Enter your vial size and dose to get a recommended mixing volume, the resulting concentration (mg/mL), and exact insulin syringe units. Works for retatrutide, tirzepatide, semaglutide, BPC-157, and any injectable peptide.

Jump to calculatorReconstitution calculatorUnits converter
Already added BAC water? Calculate concentration & syringe units.

For the “I already added water” flow, use the Peptide Units Converter and the BAC water helper to calculate concentration and syringe units.

Peptide concentration reference table

Resulting concentration (mg/mL) for common vial sizes and BAC water volumes.

Vial size+ 1 mL BAC water+ 2 mL BAC water+ 2.5 mL BAC water+ 5 mL BAC water
2 mg2.00 mg/mL1.00 mg/mL0.80 mg/mL0.40 mg/mL
3 mg3.00 mg/mL1.50 mg/mL1.20 mg/mL0.60 mg/mL
5 mg5.00 mg/mL2.50 mg/mL2.00 mg/mL1.00 mg/mL
10 mg10.00 mg/mL5.00 mg/mL4.00 mg/mL2.00 mg/mL
15 mg15.00 mg/mL7.50 mg/mL6.00 mg/mL3.00 mg/mL

How peptide reconstitution with BAC water works

Most injectable peptides are supplied as lyophilized powder — freeze-dried peptide in a sealed vial. Before use, you reconstitute the powder by injecting bacteriostatic water (BAC water) through the rubber stopper. The BAC water dissolves the powder into a clear solution ready for injection.

The reconstitution → dosing flow

  1. 1Calculate concentration: Vial mg ÷ BAC water mL = mg/mL
  2. 2Calculate volume per dose: Dose mg ÷ concentration mg/mL = mL
  3. 3Convert to syringe units: Volume mL × 100 = units (U-100 syringe)

Why BAC water? Bacteriostatic water contains 0.9% benzyl alcohol, a preservative that inhibits bacterial growth. This extends the shelf life of your reconstituted peptide to approximately 28–30 days when refrigerated, making it suitable for multi-dose vials. Sterile water (without preservative) should only be used for single injections.

Storage tip: Once reconstituted, store peptide vials refrigerated at 2–8°C (35–46°F), protected from light. Do not freeze. Discard if the solution appears cloudy, discolored, or contains particulates.

How long does BAC water last? Storage & refrigeration

Two related questions come up constantly: how long does BAC water last and does BAC water need to be refrigerated? The answer depends on what you mean — the BAC water vial itself vs a peptide vial after reconstitution.

  • BAC water (unopened): Follow the manufacturer’s label (expiration date, storage conditions).
  • BAC water (after first puncture/opening): Many multi-dose bacteriostatic water products are commonly treated as usable for ~28 days after first puncture, but you should follow the vial label and your provider’s protocol.
  • Does BAC water need to be refrigerated? Often it does not, but storage depends on the product label. In contrast, most reconstituted peptides should be refrigerated (typically 2–8°C / 35–46°F) and protected from light.
  • How to store BAC water: Keep it clean, avoid touching needles to non-sterile surfaces, and discard if contamination is suspected (cloudiness, particulates, color change).

Always follow the specific storage and beyond-use guidance provided by your prescribing clinician or compounding pharmacy.

Frequently Asked Questions

How much bacteriostatic water should I add to my peptide vial?
There isn’t one universal amount — it depends on your vial size and your dose. Use the calculator above to get a recommended BAC water volume plus alternatives (more/less water) and the resulting syringe units. Always follow the reconstitution instructions provided by your prescribing provider or compounding pharmacy.
How much BAC water for 10 mg retatrutide?
It depends on your target dose and how many units you prefer to draw. Enter your 10 mg vial and your dose in the calculator above to see a recommended BAC water amount plus other options (e.g., 1 mL vs 2 mL vs 3 mL), including the resulting concentration (mg/mL) and units per dose.
How much BAC water to mix with 30 mg tirzepatide?
The best mixing volume depends on your dose and your preferred injection volume (units). Enter your 30 mg vial and dose to get a recommended BAC water amount plus alternatives, along with concentration (mg/mL) and syringe units per dose.
What is bacteriostatic water and why is it used for peptides?
Bacteriostatic water is sterile water containing 0.9% benzyl alcohol (a preservative). The benzyl alcohol prevents bacterial growth in your reconstituted peptide solution, extending its refrigerated shelf life to approximately 28–30 days. This makes it the preferred diluent for multi-dose peptide vials.
How do I calculate my peptide concentration after adding BAC water?
Divide your vial's peptide mass (in mg) by the volume of BAC water you added (in mL). For example: 5 mg vial + 2 mL BAC water = 2.5 mg/mL. A 10 mg vial + 2 mL BAC water = 5 mg/mL. The calculator above does this automatically.
How does BAC water volume affect my syringe units per dose?
More BAC water = lower concentration = more units per dose. Less BAC water = higher concentration = fewer units per dose. For example, at 0.25 mg dose: with 2.5 mg/mL concentration you draw 10 units; with 1 mg/mL concentration you draw 25 units. Lower concentration means larger injection volumes.
How long does reconstituted peptide last in the refrigerator?
Most reconstituted peptides are stable for 28–30 days when refrigerated (2–8°C) and protected from light, provided bacteriostatic water was used. Some peptides may be more stable. Always follow your provider's guidance and discard if the solution becomes cloudy or discolored.
What is the difference between this tool and the Peptide Reconstitution Calculator?
This page answers “how much BAC water should I add?” by recommending a mixing volume from your vial size and dose, then showing the resulting concentration and syringe units. The Peptide Reconstitution Calculator includes peptide presets, cost-per-dose, and a visual syringe display for a full reconstitution workflow.

Related tools & calculators

Peptide Reconstitution Calculator

Full reconstitution calculator with peptide presets, cost-per-dose, and visual syringe display.

Peptide Units Converter

Convert any peptide dose between mg, mcg, mL, and insulin syringe units.

Semaglutide Units Converter

Dedicated semaglutide converter with common concentration presets and dosing chart.

Tirzepatide Units Converter

Convert tirzepatide doses to syringe units with a complete dosage chart.

Find a peptide-prescribing clinic

MyPeptideMatch connects you with licensed clinics that prescribe GLP-1 peptides and peptide therapy protocols. Providers supply all necessary reconstitution supplies and instructions.

Find a Clinic

Medical disclaimer: This tool is provided for informational and educational purposes only. It is not a substitute for professional medical advice, diagnosis, or treatment. Peptide reconstitution and dosing should be supervised by a licensed healthcare provider. Always follow the specific reconstitution instructions provided by your prescribing physician or compounding pharmacy.