Guides / Mixing and syringes
How much bac water to add to a peptide vial
There's no single right amount. The water sets how much peptide each unit on your syringe holds, so choose an amount that puts your dose on a line you can read.
[00] Short answer
- The amount of bacteriostatic (bac) water changes how much each syringe unit holds. It doesn't change how much peptide is in the vial.
- 2 mL is a common choice. A 5 mg vial with 2 mL is 2.5 mg/mL, so 250 mcg is the 10-unit line.
- More water spreads a dose over more units, which makes small doses easier to measure. Less water means a smaller injection.
- Pick an amount where your dose fits the syringe and lands on one of its lines. Most 1 mL syringes have a line every 2 units.
How much bac water for a 5 mg vial?
For a 5 mg vial, 2 mL of bac water makes the vial 2.5 mg/mL, so each unit on a U-100 insulin syringe holds 25 mcg and a 250 mcg dose is 10 units. With 1 mL each unit holds 50 mcg, and with 3 mL each holds about 16.7 mcg.
The table below shows the same thing for other vial sizes.
Strength after mixing, by vial size and water
Each cell shows the strength of the mixed vial and how much one unit on a U-100 syringe holds. 1 unit is 0.01 mL.
| Vial | 1 mL water | 2 mL water | 3 mL water |
|---|---|---|---|
| 5 mg | 5 mg/mL · 50 mcg per unit | 2.5 mg/mL · 25 mcg per unit | 1.67 mg/mL · 16.7 mcg per unit |
| 10 mg | 10 mg/mL · 100 mcg per unit | 5 mg/mL · 50 mcg per unit | 3.33 mg/mL · 33.3 mcg per unit |
| 15 mg | 15 mg/mL · 150 mcg per unit | 7.5 mg/mL · 75 mcg per unit | 5 mg/mL · 50 mcg per unit |
| 20 mg | 20 mg/mL · 200 mcg per unit | 10 mg/mL · 100 mcg per unit | 6.67 mg/mL · 66.7 mcg per unit |
| 30 mg | 30 mg/mL · 300 mcg per unit | 15 mg/mL · 150 mcg per unit | 10 mg/mL · 100 mcg per unit |
Water amounts that put your dose on a round line
The table gives the smallest amount of water, up to 3 mL, that puts each dose on a 10, 20, 30… unit line of a 1 mL syringe. Round numbers like 10 or 20 units are the clearest lines to read.
| Vial | 100 mcg | 250 mcg | 500 mcg | 1,000 mcg |
|---|---|---|---|---|
| 5 mg | none up to 3 mL | 2 mL → 10 units | 1 mL → 10 units | 0.5 mL → 10 units |
| 10 mg | none up to 3 mL | none up to 3 mL | 2 mL → 10 units | 1 mL → 10 units |
| 15 mg | none up to 3 mL | none up to 3 mL | 3 mL → 10 units | 1.5 mL → 10 units |
| 20 mg | none up to 3 mL | none up to 3 mL | none up to 3 mL | 2 mL → 10 units |
Should you add more water or less?
Add more water if your dose is small and hard to measure, and less if you'd rather have a smaller injection. Either way the vial holds the same total amount of peptide. Only the volume of each dose changes.
- More water means more units per dose. Small doses, like 100 mcg, become easier to measure accurately.
- Less water means fewer units per dose and a smaller injection. Very small draws, under about 2 units, are hard to read.
- Room in the vial matters too. Check your vial has space before you add more.
What is bacteriostatic water?
Bacteriostatic water is sterile water with 0.9% benzyl alcohol added. The alcohol is a preservative that slows the growth of bacteria, so one vial can be used for more than one dose1. Plain sterile water has no preservative, so it's meant for single use.
Its label says it shouldn't be used for newborns, because benzyl alcohol can be harmful to them1. The usual pharmacy rule is to use an opened multi-dose vial within 28 days unless the maker says otherwise2.
Questions
Does adding more bac water make the peptide weaker?
Not in total. Each unit holds less, but the vial still contains the same amount of peptide, so you draw more units for the same dose.
Can I use sterile water instead of bacteriostatic water?
Only for a single use. Sterile water has no preservative. Bacteriostatic water contains 0.9% benzyl alcohol, which is why it's used for vials you'll dose from more than once.
How much bac water for a 10 mg vial?
2 mL is common. It makes the vial 5 mg/mL, so each unit holds 50 mcg and a 500 mcg dose is the 10-unit line. With 1 mL a 500 mcg dose is 5 units, which is harder to read.
I already added water. Can I change it?
You can't take water out, and adding more changes every dose after it. Use the reconstitution calculator with the water you actually added.
Sources
Checked against the PeptideTracker dose math, and the Bacteriostatic Water for Injection label. How we check
- 1Bacteriostatic Water for Injection, USP (Hospira): prescribing information. description and warnings sections
- 2USP General Chapter <797> Pharmaceutical Compounding: Sterile Preparations. beyond-use dates for opened multi-dose containers
This guide explains what labels and published studies say. It isn't medical advice and doesn't replace your prescriber or pharmacist.
Spotted a mistake or something out of date? Tell us. We aim to fix factual errors within two working days.
Read next
- Reconstitution calculatorVial mg + water mL + dose → units to draw, with a syringe drawing.
- Bac water plannerHow much water makes your dose land on a round syringe line.
- How to reconstitute peptidesMixing (reconstituting) means adding bacteriostatic (bac) water to the freeze-dried powder in your vial so you can measure out doses. How much water you add, and how you add it, sets every dose that follows.