How much bacteriostatic water should I add to a 5 mg peptide vial?

There is no single correct answer — it depends on your desired concentration and dose. Adding 1 mL to a 5 mg vial gives you 5 mg/mL (5000 mcg/mL). Adding 2 mL gives you 2.5 mg/mL (2500 mcg/mL). More water means a lower concentration and larger injection volumes, but easier measurement of small doses. Use the calculator above to find the best ratio for your protocol.

Can I use this calculator for semaglutide and tirzepatide?

Yes. This calculator works for any peptide that requires reconstitution — including semaglutide, tirzepatide, BPC-157, TB-500, CJC-1295, ipamorelin, and all other lyophilized research peptides. Just enter the vial size in mg, the amount of water you added, and your desired dose.

Peptide Reconstitution Calculator

How to Use This Calculator

Three inputs, instant results. Here's how it works:

Peptide Amount -- Enter the total amount of peptide in your vial. This is printed on the label (e.g., 5 mg, 10 mg). Toggle between mg and mcg if your vial is labeled in micrograms.
BAC Water Volume -- Enter how much bacteriostatic water you added (or plan to add) to the vial. Common amounts are 1 mL, 2 mL, or 3 mL. Toggle between mL and cc (they're the same).
Desired Dose -- Enter the dose you want per injection. Toggle between mg and mcg depending on your protocol.

The calculator instantly shows your concentration, injection volume in mL, equivalent syringe units on a standard 100-unit insulin syringe, and how many total doses you'll get from the vial.

Understanding the Math

Reconstitution math is straightforward once you see the formula. There are really only two steps:

Step 1: Find your concentration. Divide the total peptide amount by the water volume. That gives you the concentration in mg/mL (or mcg/mL if you multiply by 1,000).

Step 2: Find your injection volume. Divide your desired dose by the concentration. That tells you how many mL to draw. Multiply by 100 to convert to insulin syringe units (since 1 mL = 100 units on a standard insulin syringe).

Worked Example -- BPC-157

You have a 5 mg vial of BPC-157 and add 2 mL of bacteriostatic water. Your target dose is 250 mcg.

Concentration: 5 mg / 2 mL = 2.5 mg/mL = 2,500 mcg/mL

Injection volume: 250 mcg / 2,500 mcg/mL = 0.1 mL

Syringe units: 0.1 mL x 100 = 10 units on an insulin syringe

Doses per vial: 5,000 mcg / 250 mcg = 20 doses

Common Reconstitution Volumes

The table below shows concentrations you'll get for common vial sizes and water volumes. Use less water for a more concentrated solution (smaller injection volumes), or more water for easier measurement of small doses.

Vial Size	1 mL Water	2 mL Water	3 mL Water
2 mg	2 mg/mL (2,000 mcg/mL)	1 mg/mL (1,000 mcg/mL)	0.67 mg/mL (667 mcg/mL)
5 mg	5 mg/mL (5,000 mcg/mL)	2.5 mg/mL (2,500 mcg/mL)	1.67 mg/mL (1,667 mcg/mL)
10 mg	10 mg/mL (10,000 mcg/mL)	5 mg/mL (5,000 mcg/mL)	3.33 mg/mL (3,333 mcg/mL)
15 mg	15 mg/mL (15,000 mcg/mL)	7.5 mg/mL (7,500 mcg/mL)	5 mg/mL (5,000 mcg/mL)

Frequently Asked Questions

How do I reconstitute a peptide vial?

Draw your chosen volume of bacteriostatic water into a syringe, then inject it slowly down the inside wall of the peptide vial. Don't spray directly onto the lyophilized powder. Let it dissolve on its own or gently swirl the vial -- never shake it. Most peptides dissolve within a few minutes. For a full walkthrough, see our Getting Started guide.

How many units on an insulin syringe is 0.1 mL?

On a standard 100-unit (1 mL) insulin syringe, 0.1 mL equals 10 units. Each small tick mark represents 1 unit (0.01 mL). So 0.25 mL = 25 units, 0.5 mL = 50 units, and 1 mL = 100 units. If you're new to insulin syringes, our insulin syringe guide covers everything.

How much bacteriostatic water should I add?

It depends on your desired concentration and dose. Adding less water gives a more concentrated solution (smaller injections), while more water makes it easier to measure small doses precisely. For most peptides in the 5-10 mg range, 1-2 mL is standard. Our bacteriostatic water guide covers this in detail.

What's the difference between bacteriostatic water and sterile water?

Bacteriostatic water contains 0.9% benzyl alcohol, which inhibits bacterial growth. This allows the vial to be used for multiple draws over up to 28 days. Sterile water has no preservative and is single-use only. For multi-dose peptide vials, bacteriostatic water is the standard choice.

Does this calculator work for semaglutide and tirzepatide?

Yes. This calculator works for any peptide that requires reconstitution -- semaglutide, tirzepatide, BPC-157, TB-500, CJC-1295, ipamorelin, and every other lyophilized research peptide. Just enter your vial size, water volume, and target dose.