One of the most common questions from anyone new to peptide research is: how much bacteriostatic water do I add to my vial? Get this wrong and every dose you measure will be off. This guide walks you through the exact maths — and we've built a free calculator below to do it for you instantly.
Understanding Peptide Concentration
When you receive a lyophilised (freeze-dried) peptide vial, the powder inside has a specific weight — usually expressed in milligrams (mg). To use the peptide, you need to dissolve it in bacteriostatic water (BAC water) to create a liquid solution. The concentration of that solution depends entirely on how much water you add.
The formula is straightforward:
Concentration (mcg/mL) = Peptide Amount (mcg) ÷ Volume of BAC Water Added (mL)
For example, if you add 2mL of BAC water to a 10mg vial:
- 10mg = 10,000 mcg
- 10,000 mcg ÷ 2mL = 5,000 mcg/mL
Every millilitre of the resulting solution now contains 5,000 micrograms of peptide.
Why the Amount of Water Matters
Adding more water lowers the concentration, which means you need to draw a larger volume to achieve the same dose. Adding less water raises the concentration, so smaller volumes deliver more peptide. Neither is inherently wrong — it comes down to what dose you need and how precisely your syringe can measure it.
For most research applications, a concentration that puts your target dose somewhere between 5 and 20 units on a U-100 insulin syringe is ideal. This range is easy to measure accurately without being too small to read.
Reading a U-100 Insulin Syringe
Most researchers use U-100 insulin syringes — the same ones included in our 100pk syringe packs. Here is what you need to know:
- A U-100 syringe has 100 units per mL
- Each unit = 0.01mL
- So 10 units = 0.10mL, 20 units = 0.20mL, and so on
To calculate how many units to draw for a specific dose:
Units to draw = Desired Dose (mcg) ÷ (Concentration (mcg/mL) ÷ 100)
Example: You want a 250mcg dose from a 5,000 mcg/mL solution:
- 5,000 ÷ 100 = 50 mcg per unit
- 250 ÷ 50 = 5 units
Use Our Free Peptide Calculator
Enter your vial size, how much BAC water you plan to add, and your target dose — the calculator will tell you exactly how many units to draw.