How much bacteriostatic water do I add to a peptide vial?

It depends on your target concentration. For a 5mg vial at 500mcg/mL, add 10mL of bacteriostatic water. The formula is: BAC water (mL) = vial size (mcg) ÷ target concentration (mcg/mL). Use the calculator above to compute this automatically for any combination.

What is bacteriostatic water?

Bacteriostatic water (BAC water) is sterile water preserved with 0.9% benzyl alcohol. The preservative prevents bacterial growth, making it suitable for multi-dose reconstitution. It is the standard diluent for most research peptides.

How many units do I draw on a U-100 syringe?

On a U-100 (100-unit insulin) syringe, 1mL equals 100 units. To find your units: dose (mcg) ÷ concentration (mcg/mL) = volume (mL), then multiply by 100. For a 250mcg dose at 500mcg/mL: 0.5mL × 100 = 50 units.

Peptide Calculator

Peptide Reconstitution Calculator

Calculate exactly how much bacteriostatic water to add to your peptide vial, and how many mL or syringe units to draw per dose. Works for any lyophilised (freeze-dried) peptide. Free, instant, no login.

How to use this calculator

You need your vial size in milligrams (printed on the vial — e.g., 5mg), your target concentration in mcg/mL, and your dose in mcg per injection.

Enter the vial size and target concentration first — the calculator shows how much bacteriostatic water to inject into the vial. Then enter your dose per injection to see exactly how many mL and how many U-100 syringe units to draw each time.

If you are unsure what concentration to use, a common starting point for a 5mg vial is 500mcg/mL, which requires 10mL of BAC water. This gives manageable injection volumes for doses in the 250–500mcg range.

Worked example — BPC-157 5mg vial

Reconstitution

Vial size: 5 mg (5,000 mcg)

Target concentration: 500 mcg/mL

BAC water to add = 5,000 ÷ 500 = 10 mL

Per-injection dose — 250 mcg

Concentration: 500 mcg/mL

Dose: 250 mcg

Draw = 250 ÷ 500 = 0.50 mL = 50 units on a U-100 syringe

How to interpret your result

The BAC water volume is the amount of bacteriostatic water to inject slowly into the vial using a sterile syringe. Inject it down the side of the vial — never directly onto the peptide powder — and swirl gently. Do not shake.

The units result is what you read on a U-100 (100-unit insulin) syringe. U-100 syringes are marked in units, not mL — 100 units equals 1mL, so 50 units equals 0.5mL.

Store reconstituted peptides refrigerated at 2–8°C. Most peptides are stable for 30 days once reconstituted, though stability varies — check the specific data for your peptide.

Common reconstitution volumes for a 5mg vial

BAC water added	Concentration	250 mcg dose	500 mcg dose
2 mL	2,500 mcg/mL	10 units	20 units
5 mL	1,000 mcg/mL	25 units	50 units
10 mL	500 mcg/mL	50 units	100 units

Frequently asked questions

Does it matter how much BAC water I add?

Yes — the amount determines your concentration, which directly affects how many units you draw per dose. Using less BAC water gives a higher concentration and smaller injection volumes. Using more gives a lower concentration and larger volumes. There is no single correct amount; it depends on your dose range and syringe preference.

Can I use sterile water instead of bacteriostatic water?

Sterile water has no preservative, so it is only suitable for single-use reconstitution. Bacteriostatic water (with 0.9% benzyl alcohol) is preferred for multi-dose vials because it inhibits bacterial growth over multiple punctures.

How long does reconstituted peptide last?

Most peptides are stable for approximately 30 days when refrigerated at 2–8°C after reconstitution with bacteriostatic water. Stability varies by peptide — some are more fragile. Freeze the vial before reconstitution for longer storage.

What is a U-100 syringe?

A U-100 syringe is a standard insulin syringe calibrated for U-100 insulin (100 units per mL). The unit markings on the barrel correspond directly: 100 units = 1mL, 50 units = 0.5mL. They are commonly used for subcutaneous peptide injections because of the small gauge needle and precise markings.

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