Why the amount of water matters
The water does not change how much peptide is in the vial; it changes how spread out it is. That decides how much liquid each dose takes up in the syringe. Too little water and each dose is a tiny volume where a small misreading is a large error. Too much and doses become bulky, or the vial overflows.
A practical goal is to make your usual dose land on a round, easy-to-read number of units, such as 10 or 20, on the syringe you already use. Work backwards from that number and you get the water volume.
Water to add (mL) = units per dose × vial amount (mcg) ÷ (dose (mcg) × 100)
Example: 10 units × 5,000 mcg ÷ (250 mcg × 100) = 2 mL
Quick mixing table
Water needed so that the dose in each row is exactly 10 units (0.1 mL) on a U-100 syringe.
| Dose | 2 mg vial | 5 mg vial | 10 mg vial |
|---|---|---|---|
| 100 mcg | 2 mL | 5 mL | over 5 mL |
| 250 mcg | 0.8 mL | 2 mL | 4 mL |
| 500 mcg | 0.4 mL | 1 mL | 2 mL |
| 1 mg | 0.2 mL | 0.5 mL | 1 mL |
Mixing tips
- Let the vial and water reach room temperature first, and clean both stoppers with an alcohol swab.
- Add the water slowly, aiming it at the inside wall of the vial rather than straight onto the powder.
- Swirl or roll the vial gently until it dissolves. Do not shake it hard.
- Label the vial with the date and the concentration, for example "2,500 mcg/mL".
The complete process is in how to reconstitute peptides. Once mixed, use the reconstitution calculator or the dosage calculator to read any dose.
BAC water questions
What is bacteriostatic water?
Sterile water that contains 0.9% benzyl alcohol as a preservative. The preservative slows the growth of bacteria, which is why it is used for vials that are punctured more than once. Plain sterile water has no preservative and is meant for single use.
How much bacteriostatic water do I add to a 5 mg vial?
Common choices are 1, 2 or 2.5 mL, and none is the "right" one by itself. With 2 mL, each unit holds 25 mcg; with 2.5 mL, each unit holds 20 mcg. Pick the volume that makes your dose a convenient number of units, which is exactly what this calculator does.
How much water for a 10 mg vial?
With 2 mL, each unit is 50 mcg; with 3 mL, about 33.3 mcg. If your dose is 500 mcg, 2 mL makes it 10 units, a clean number to measure.
Can I add more water later?
Adding more liquid to an already-mixed vial changes its concentration, so every dose calculation must be redone. It is simpler and less error-prone to choose the volume once, before mixing, and write it on the vial.
Is there a maximum amount of water?
The vial itself limits you. Many small peptide vials hold around 3 mL, and overfilling makes the stopper hard to puncture cleanly. The calculator warns you if the amount is large.