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 (mg/mL) = Peptide Amount (mg) ÷ Volume of BAC Water Added (mL)
For example, if you add 2 mL of BAC water to a 10 mg vial:
- 10 mg ÷ 2 mL = 5 mg/mL
Every millilitre of the resulting solution now contains 5 mg 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-50 insulin syringe is ideal. This range is easy to measure accurately without being too small to read.
Reading a U-50 Insulin Syringe
Most researchers use U-50 insulin syringes — the same ones included in our 20pk syringe packs. Here is what you need to know:
- A U-50 syringe holds 50 units total (0.5 mL barrel)
- Each unit = 0.01 mL
- So 10 units = 0.10 mL, 20 units = 0.20 mL, and so on
To calculate how many units to draw for a specific dose:
Units to draw = Desired Dose (mg) ÷ (Concentration (mg/mL) ÷ 100)
Example: You want a 0.25 mg dose from a 5 mg/mL solution:
- 5 ÷ 100 = 0.05 mg per unit
- 0.25 ÷ 0.05 = 5 units
Use Our Free Peptide Calculator
Most peptide calculators ask the wrong question. They ask how much water you're adding — but in practice, what you actually know is your vial size and your target dose. The water amount should be the output, not the input.
Our calculator works this way: select your peptide, enter your target dose, and it calculates exactly how much BAC water to add so your dose lands on a real syringe tick mark — no guessing, no 17.3-unit draws.
