The Easiest Way to Calculate Your Peptide Dose
Staring at a peptide vial, a bottle of bacteriostatic water, and an insulin syringe without knowing what to do with any of them is one of the most common stopping points for people who are otherwise completely ready to start a protocol.
The confusion is not about the peptide. It is about the math, and specifically about the relationship between three things: how much powder is in the vial, how much water you add, and what that combination means for every unit marking on your syringe.
Start with the water.
When a peptide arrives in a vial, it is in a lyophilized form, which means it has been freeze-dried into a powder so it stays stable during storage and shipping. Before you can inject it, you have to reconstitute it, which means dissolving that powder back into a liquid using something called bacteriostatic water, which is sterile water preserved with a small amount of benzyl alcohol that prevents bacterial growth so the vial stays usable across multiple draws over time.
The question most people get wrong is how much bacteriostatic water to add.
The instinct is to use less. Less water means more concentrated, and more concentrated sounds like it means you draw smaller volumes, which feels more precise. But this logic breaks down in practice because peptide powder does not always dissolve completely in small volumes of liquid, and a peptide that has not fully dissolved means your doses are inconsistent even if your unit count is exactly right.
The standard that resolves this is 2 mL of bacteriostatic water per vial for the small 3 mL vials that most peptides come in. Two milliliters gives the powder enough fluid to dissolve completely and gives you a total solution volume that makes the dosing math clean and repeatable. This is the number you fix, and you do not change it, because changing it every time would mean recalculating every time.
Now the math, and it is simpler than it looks.
An insulin syringe is marked in units, and those units refer to insulin units, which is just a volume measurement that equals one hundredth of a milliliter. So 100 units on a syringe equals 1 mL of fluid. That is all the syringe is doing, measuring volume in a very small and precise way.
When you add 2 mL of bacteriostatic water to a vial, you now have 2 mL of total solution, and everything in that vial is evenly distributed across those 2 mL. So if your vial contains 10 milligrams of peptide and you added 2 mL of water, you have 10 milligrams spread across 2 mL, which means every 1 mL contains 5 milligrams, and every 0.1 mL, which is 10 units on the syringe, contains 0.5 milligrams, which is 500 micrograms.
That one calculation, done once for three common vial sizes, gives you everything you need.
A 5 milligram vial in 2 mL means every 10 units is 250 micrograms.
A 10 milligram vial in 2 mL means every 10 units is 500 micrograms.
A 20 milligram vial in 2 mL means every 10 units is 1 milligram, which is 1,000 micrograms.
From there, you are not calculating anything new. You are just scaling the number you already know. If you have a 10 milligram vial and your dose is 250 micrograms, that is half of 500, so you draw to 5 units. If your dose is 1 milligram, that is double 500, so you draw to 20 units. The anchor number for your vial size stays fixed, and every dose is just a multiple or fraction of it.
The reason this system works is because you held the water volume constant at 2 mL. If you had added 1.5 mL on one reconstitution and 2.5 mL on another, your anchor numbers would shift every time and the simple scaling would no longer apply. Consistency in the one variable you control, which is how much water you add, is what keeps the downstream math stable.
There is one thing worth understanding about why bacteriostatic water specifically and not sterile water. Regular sterile water has no preservative, which means once you puncture the vial with a needle, bacteria can potentially enter, and any remaining solution degrades quickly. Bacteriostatic water contains 0.9 percent benzyl alcohol, which inhibits bacterial growth and allows a multi-dose vial to be used safely over a period of weeks when stored properly in the refrigerator. This is the standard referenced in compounding guidelines for multi-dose injectable preparations, and it is why bacteriostatic water is the appropriate choice for peptide vials that you will be drawing from repeatedly rather than using all at once.
The practical upshot is that once you have your vial size, your water volume locked at 2 mL, and your anchor number for that vial size, dosing becomes a simple lookup rather than a calculation. You know your dose, you know your anchor, and you draw the corresponding units.
Most dosing errors with peptides come not from getting the drug wrong but from inconsistent reconstitution, where someone adds a different volume of water each time, which shifts the concentration each time, which means drawing the same number of units produces a different dose each time without the person realizing it.
Fixing the water volume at 2 mL is not just a convenience. It is the step that makes every other part of the process reliable.
References
- United States Pharmacopeia. USP General Chapter 797: Pharmaceutical Compounding, Sterile Preparations. Establishes compounding standards for reconstitution of lyophilized injectable compounds, including multi-dose vial protocols and bacteriostatic water usage. Source
Join the free community:
Men: Iron Forge Brotherhood
Women: Powerhouse Fitness
If this is the kind of information you want access to on a daily basis, the community is free and there are full courses on training, nutrition, hormones, and supplementation inside. You can ask questions and post your own labs and get feedback from me and from the community.