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Peptide Reconstitution Calculator: Precision BAC Water Math
Lyophilized peptide vials arrive as a dry powder that cannot be injected until dissolved in a sterile diluent. Reconstitution — adding a precise volume of bacteriostatic water (BAC water) — determines three critical variables: solution concentration (mg/mL), draw volume per dose (mL), and the equivalent units on a U-100 insulin syringe. This free calculator handles the math instantly for tirzepatide, semaglutide, BPC-157, ipamorelin, CJC-1295, TB-500, and 20+ lyophilized peptides.
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Download on App StoreHow to Use This Calculator
Enter three numbers and read four results. The calculator updates live — no submit button needed.
- 1Select your compound (optional) — choosing a preset auto-fills the typical vial size, starting dose, and dose unit. You can override any field manually.
- 2Enter vial peptide amount (mg) — this is printed on your vial label. Common sizes: 2 mg, 5 mg, 10 mg.
- 3Enter BAC water volume (mL) — the amount of bacteriostatic water you plan to add. This is your key variable. Start with 1–2 mL and adjust based on the output draw volume.
- 4Enter your target dose — in mg or mcg. Switch the dose unit dropdown to match your protocol. Example: BPC-157 doses are typically in mcg (250–500 mcg); tirzepatide doses are in mg (2.5–15 mg).
- 5Select your syringe type — U-100 1 mL is standard for most peptides. U-100 0.5 mL and 0.3 mL provide finer graduation marks for small-volume draws.
- 6Read the four results: Concentration (mg/mL), Volume in mL (how much to draw), Syringe units (the number you read off the barrel), and Doses remaining (total doses the vial yields).
What Is Bacteriostatic Water (BAC Water)?
Bacteriostatic water is sterile water for injection that contains 0.9% benzyl alcohol (9 mg/mL) as an antimicrobial preservative. The benzyl alcohol inhibits bacterial growth through disruption of microbial cell membranes, allowing a multi-use vial to remain sterile across repeated draws for 28–30 days under refrigeration. This makes it the gold-standard reconstitution diluent for research peptides, which are typically administered via repeated subcutaneous injections over weeks to months.
Bacteriostatic water is isotonic, pH-compatible with most lyophilized peptides, and widely available in 30 mL multi-dose vials. It is distinct from plain sterile water for injection (which lacks a preservative and must be discarded within 24 hours of first puncture) and from normal saline (0.9% NaCl), which is unpreserved in most commercial presentations.
BAC Water vs. Other Reconstitution Diluents
| Diluent | Preservative | Vial Life After Reconstitution | Best Use Case |
|---|---|---|---|
| Bacteriostatic Water (BAC) | Benzyl alcohol 0.9% | 28–30 days (refrigerated) | Most research peptides, GLP-1 agonists, GH secretagogues |
| Sterile Water for Injection | None | Discard within 24 hours | Single-dose protocols, benzyl alcohol sensitivity |
| Normal Saline (0.9% NaCl) | None | Discard within 24 hours | Tesamorelin (Egrifta FDA label), some hospital protocols |
| Bacteriostatic Normal Saline | Benzyl alcohol 0.9% | 28–30 days (refrigerated) | Peptides requiring tonicity matching |
| Acetic Acid (0.1%) in Water | None (low pH inhibits growth) | Up to 7 days (refrigerated) | Hydrophobic peptides poorly soluble at neutral pH |
How Long Does a Reconstituted Peptide Vial Last?
A peptide vial reconstituted with BAC water should be refrigerated immediately at 2–8°C (36–46°F) and used within 28–30 days. Unreconstituted lyophilized peptides are typically stable for 12–24 months at room temperature, and up to 36+ months when frozen. Once reconstituted, the clock starts regardless of whether BAC water was used.
- Refrigerate at 2–8°C immediately after reconstitution — do not leave at room temperature
- Do not freeze a reconstituted vial: benzyl alcohol can precipitate, and freeze-thaw cycles degrade peptide bonds
- Allow to reach room temperature naturally (15–20 min) before drawing — do not heat the vial
- Discard if the solution appears cloudy, discolored, or contains visible particles
- Label each vial with the reconstitution date and discard after 28–30 days regardless of remaining volume
The Reconstitution Formula — Three Steps
Every peptide reconstitution calculation uses the same three-formula chain. Understanding the math lets you adapt to any vial size, dose, or syringe.
On a U-100 insulin syringe, 1 mL = 100 units. This is a fixed relationship — the syringe barrel is marked in units but each unit equals 0.01 mL. So 10 units = 0.10 mL, 20 units = 0.20 mL, and so on.
Worked Examples by Compound
| Compound | Vial | BAC Water | Concentration | Typical Dose | Draw (mL) | U-100 Units |
|---|---|---|---|---|---|---|
| Tirzepatide | 5 mg | 2 mL | 2.5 mg/mL | 2.5 mg | 1.00 mL | 100 U |
| Semaglutide | 3 mg | 2 mL | 1.5 mg/mL | 0.25 mg | 0.167 mL | 17 U |
| BPC-157 | 5 mg | 2 mL | 2.5 mg/mL | 250 mcg | 0.10 mL | 10 U |
| Ipamorelin | 5 mg | 2.5 mL | 2.0 mg/mL | 200 mcg | 0.10 mL | 10 U |
| CJC-1295 (no DAC) | 5 mg | 2.5 mL | 2.0 mg/mL | 100 mcg | 0.05 mL | 5 U |
| TB-500 | 5 mg | 2 mL | 2.5 mg/mL | 2.5 mg | 1.00 mL | 100 U |
| Sermorelin | 9 mg | 3 mL | 3.0 mg/mL | 200 mcg | 0.067 mL | 6.7 U |
| PT-141 | 10 mg | 2 mL | 5.0 mg/mL | 1.75 mg | 0.35 mL | 35 U |
How Much BAC Water to Add — Concentration Strategy
The volume of bacteriostatic water you add controls your solution concentration, which in turn determines draw volume and dose precision. There is no universally "correct" volume — the optimal amount depends on your target dose and syringe graduation resolution.
Low vs. High Concentration: Trade-offs
| BAC Water Volume (5 mg vial) | Concentration | Draw for 2.5 mg dose | Advantage | Disadvantage |
|---|---|---|---|---|
| 1 mL | 5.0 mg/mL | 0.5 mL / 50 U | Smaller injection volume | Small draws hard to measure for mcg doses |
| 2 mL (most common) | 2.5 mg/mL | 1.0 mL / 100 U | Easy to read; fits 1 mL syringe | Full syringe per dose if dose is large |
| 5 mL | 1.0 mg/mL | 2.5 mL / 250 U | Very precise for small mcg doses | High volume per injection; exceeds standard syringes |
Minimum Viable Draw Volume
A U-100 insulin syringe has graduation marks every 1 unit (0.01 mL). In practice, draws below 5 units (0.05 mL) introduce meaningful measurement error because the meniscus of the liquid occupies the same visual space as several graduation marks. If your draw volume is calculated as less than 5 units, add more BAC water to your next vial to lower the concentration and increase the draw volume. For peptides with very small doses (e.g., 50–100 mcg of CJC-1295 no-DAC), a 0.3 mL syringe with finer graduations improves accuracy further.
Peptide Storage After Reconstitution
Lyophilized peptides are chemically stable in their dried state. Once reconstituted, peptide degradation is accelerated by heat, UV light, repeated freeze-thaw cycling, and microbial contamination. Proper storage preserves peptide potency and extends usable vial life.
- Refrigerate at 2–8°C (36–46°F) immediately after reconstitution and between uses
- Do not freeze a reconstituted vial — benzyl alcohol can precipitate, and freeze-thaw cycles denature peptide tertiary structure
- Protect from light — store vials in original packaging or a dark drawer; UV exposure accelerates peptide bond degradation
- Allow to reach room temperature by leaving out for 15–20 minutes before drawing — do not use a microwave or warm water
- Use aseptic technique — wipe the vial stopper with a 70% isopropyl alcohol swab before each draw; use a clean needle and syringe each time
- Label each vial with the reconstitution date; discard after 28–30 days even if volume remains
Injection Site Rotation and Lipohypertrophy
Subcutaneous peptide injections should be rotated across multiple anatomical sites — abdomen, thighs, upper arms — to prevent lipohypertrophy: localized thickening and fibrosis of subcutaneous fat tissue caused by repeated injections at the same site. Lipohypertrophy is a recognized clinical complication associated with impaired drug absorption; studies of insulin-using patients show that injecting into lipohypertrophic tissue reduces pharmacokinetic predictability and can cause erratic blood concentration profiles.
Allow at least 7 days before returning to any previously injected site. Use the Halflife Injection Site Rotation Tracker to log and visualize your rotation schedule.
Frequently Asked Questions
How much BAC water do I add to a 5 mg peptide vial?
There is no single required amount — the correct volume depends on your target dose and desired draw volume. Adding 1 mL yields 5 mg/mL (small draws, suitable if your dose is 1–2.5 mg). Adding 2 mL yields 2.5 mg/mL (the most common starting point for GLP-1 peptides). Adding 5 mL yields 1 mg/mL (easiest to measure for very small mcg doses). Use the calculator above with your target dose to find the right volume — aim for a draw between 0.05 mL and 0.5 mL.
Does adding more BAC water weaken the peptide or reduce the dose?
No. Adding more bacteriostatic water lowers the concentration (mg/mL) but does not reduce the total amount of peptide in the vial. If your vial contains 5 mg of tirzepatide, it contains 5 mg regardless of whether you add 1 mL or 5 mL of water. You simply draw a proportionally larger volume per dose. Total active compound = unchanged. Dilution affects draw volume only — not dose strength per injected volume.
What happens if I accidentally add too much water to a peptide vial?
The peptide is not ruined. You now have a lower-concentration solution, so you will draw a larger volume per dose. Recalculate using the actual total volume you added. As long as you have not contaminated the vial (used a clean needle, sterile technique), the peptide remains usable. If the total volume now exceeds your syringe capacity for a single dose, consult the calculator and consider drawing two injections for a single dose, or use a larger syringe.
How do I convert mL to units on a U-100 insulin syringe?
Multiply mL × 100. On a U-100 syringe, the scale is defined so that 1 mL = 100 units. This is a mathematical identity — 1 unit = 0.01 mL, always. Examples: 0.05 mL = 5 units · 0.1 mL = 10 units · 0.2 mL = 20 units · 0.5 mL = 50 units · 1.0 mL = 100 units. A U-40 syringe uses a different scale (1 mL = 40 units) — switch the syringe dropdown in the calculator to U-40 if you use one.
How do I calculate a 250 mcg BPC-157 dose in syringe units?
Step 1: Calculate concentration. If you added 2 mL BAC water to a 5 mg vial: 5 ÷ 2 = 2.5 mg/mL. Step 2: Convert dose to mg: 250 mcg = 0.25 mg. Step 3: Calculate draw volume: 0.25 ÷ 2.5 = 0.10 mL. Step 4: Convert to units: 0.10 × 100 = 10 units on U-100. For a 500 mcg dose at the same concentration: 20 units.
What is the standard BAC water protocol for compounded tirzepatide?
A common research protocol for compounded tirzepatide is 2 mL BAC water per 5 mg vial, yielding 2.5 mg/mL. At this concentration: 2.5 mg dose = 1.0 mL = 100 U on U-100 · 5 mg dose = 2.0 mL (requires two draws or a 2 mL syringe) · 1.25 mg (half-dose) = 0.5 mL = 50 U. Note: FDA-approved Mounjaro and Zepbound are pre-filled, single-use injection pens and do not require reconstitution. The above applies to compounded tirzepatide only.
Can I use saline instead of bacteriostatic water?
Unpreserved normal saline (0.9% NaCl) can be used but must be discarded within 24 hours of first use because it contains no antimicrobial preservative. Bacteriostatic saline (0.9% NaCl + 0.9% benzyl alcohol) extends vial life to 28–30 days and is an acceptable alternative to BAC water for most peptides. Tesamorelin (Egrifta) is specifically labeled for reconstitution with sterile water for injection (not bacteriostatic water) — always follow the prescribing information for FDA-approved products.
How long does a reconstituted peptide vial last in the refrigerator?
When reconstituted with bacteriostatic water and refrigerated at 2–8°C (36–46°F), most research peptides remain stable for 28–30 days. This window assumes aseptic handling: the vial stopper was swabbed with 70% isopropyl alcohol before each draw, and a fresh needle was used each time. Beyond 30 days, the antimicrobial protection of benzyl alcohol may be insufficient to guarantee sterility across multiple punctures. Discard the vial regardless of remaining volume.
Why won't my peptide dissolve in bacteriostatic water?
Some lyophilized peptides require more time or gentle agitation to fully dissolve. Roll the vial slowly between your palms for 30–60 seconds — never shake vigorously, as mechanical shear forces can denature peptide tertiary structure. If the peptide remains partially undissolved, refrigerate for 15 minutes then roll again. Very hydrophobic peptides (certain melanocortin analogues, lipidated derivatives) may require a small volume of co-solvent such as 0.1% acetic acid added first before diluting to final volume with BAC water.
Is it safe to use the same vial of bacteriostatic water for multiple peptides?
BAC water vials are multi-dose — the 0.9% benzyl alcohol preservative is specifically designed for repeated needle punctures. Using one BAC water vial as a source to reconstitute multiple peptide vials is standard practice, provided you use a clean, sterile needle each time you draw from the BAC water vial. Never inject directly from the BAC water vial — draw the required volume into a syringe and inject from the syringe into the peptide vial.
What is the difference between a U-40 and U-100 syringe for peptide injections?
U-40 syringes are calibrated to 40 units per mL (1 mL = 40 units). U-100 syringes are calibrated to 100 units per mL (1 mL = 100 units). U-100 is the standard for research peptides. If you use a U-40 syringe with U-100 calculations, you will inject 2.5× the intended dose — a serious error. Always confirm your syringe type and use the matching syringe type selector in this calculator. U-100 insulin syringes are the most widely available and the default assumption for all peptide reconstitution protocols.
Related Tools & Compounds
Educational calculator only — not medical advice. All calculations are for informational purposes. Verify all doses, volumes, and syringe readings against your prescriber's instructions and product labeling before administering any injectable. Reconstituted peptide protocols for research compounds are not FDA-evaluated for safety or efficacy. Methodology · Terms