Peptides arrive as a dry powder in a sealed vial. Before they can be injected, they need to be dissolved in liquid. That liquid is almost always bacteriostatic water (commonly called bac water), which is sterile water with a small amount of benzyl alcohol added to prevent bacterial growth and extend the shelf life of the reconstituted solution.
Once reconstituted, the vial holds a known concentration of peptide. The question then becomes: how many units do you pull into a syringe to get a specific dose? That's a math problem, and it has a clean answer.
What you need to know before you calculate
Three numbers:
- Vial size: how many milligrams (mg) of peptide are in the vial (e.g., 5 mg, 10 mg)
- Bac water volume added: how many milliliters (mL) of bac water you add to reconstitute (e.g., 1 mL, 2 mL)
- Desired dose: how many mcg or mg you intend to draw (this comes from your own research or a provider)
How much bac water should you add?
There's no single right answer. The amount of bac water you add determines the concentration of your solution. A smaller volume means a more concentrated solution; a larger volume means a more dilute one.
The most common reason to choose one over the other is the syringe you're using and the dose you're drawing. If your dose is very small (say, 100 mcg), you want enough volume in the solution that you can measure it accurately on a syringe. If you use too little bac water, 100 mcg might only be 2 or 3 units on a syringe, which is hard to measure precisely. More bac water spreads the peptide out, so your dose takes up more space on the syringe and is easier to draw accurately.
Most people working with peptides in the 100 mcg to 500 mcg range find that 1 mL or 2 mL of bac water per vial gives a workable concentration. For milligram-range doses, you may be fine with 1 mL. These are practical considerations, not chemistry requirements: the math below works regardless of how much bac water you choose.
The concentration formula
Once you've added your bac water, your concentration is simply:
Concentration = vial size (in mcg) divided by bac water volume (in mL)
For example: a 5 mg vial reconstituted with 2 mL of bac water.
- 5 mg = 5,000 mcg
- 5,000 mcg divided by 2 mL = 2,500 mcg per mL
Your solution now contains 2,500 mcg per every 1 mL.
Translating mL to syringe units
Most insulin syringes used for peptide injections are marked in units, not milliliters. A standard 100-unit insulin syringe holds 1 mL total. So 100 units = 1 mL, which means:
- 10 units = 0.1 mL
- 20 units = 0.2 mL
- 50 units = 0.5 mL
- 100 units = 1 mL
Putting it together: a worked example
Let's say you have a 10 mg vial, you add 2 mL of bac water, and you want to draw a 500 mcg dose.
Step 1: Find your concentration.
10 mg = 10,000 mcg. Divided by 2 mL = 5,000 mcg per mL.
Step 2: Find what fraction of a mL equals your dose.
500 mcg divided by 5,000 mcg/mL = 0.1 mL.
Step 3: Convert to syringe units.
0.1 mL = 10 units on a 100-unit syringe.
So you'd draw to the 10-unit mark. That's your 500 mcg dose.
A second example with a smaller dose
Same 10 mg vial, same 2 mL bac water (5,000 mcg/mL), but now you want 250 mcg.
- 250 mcg divided by 5,000 mcg/mL = 0.05 mL
- 0.05 mL = 5 units on a 100-unit syringe
Five units is a small but readable mark on a syringe. If you found that too difficult to measure accurately, you could add more bac water (say, 4 mL instead of 2 mL) to spread the concentration out and make the same dose occupy 10 units instead of 5.
The formula, restated simply
Once you know your concentration in mcg/mL, every calculation follows the same pattern:
Units to draw = (dose in mcg divided by concentration in mcg/mL) multiplied by 100
That final multiplication by 100 converts mL to units on a 100-unit syringe.
Where to go from here
If you're still working out what dose range to be looking for in the first place, How Peptide Dosing Is Communicated in Research explains the vocabulary: mcg/kg, protocol descriptions, and why dosing varies between compounds.
For context on specific peptides, the research notes pages on this site document the dose ranges used in published studies. BPC-157, TB-500, and MOTS-c all include protocol information from the literature, which is useful context when you're trying to understand what researchers have actually tested.