How Much Bacteriostatic Water Should You Add?
Written and reviewed by Evan Marlow, developer of DosePlot · Updated 2026-08-27
There isn't one correct amount. The water volume you add sets the concentration of the finished vial, and that decides how large each dose's draw is on the syringe. The peptide amount never changes — you're only choosing how spread out it is, and therefore how easy the dose is to measure.
What the water volume actually changes
Concentration is simply the peptide mass divided by the water you add. Add more water and the concentration drops, so the same dose becomes a larger volume — more units on the barrel, easier to read accurately. Add less water and the dose shrinks to a few hard-to-read units. Here is the same 5 mg vial at four common volumes, with the draw for a 1 mg example dose:
| Water added | Concentration | A 1 mg dose draws |
|---|---|---|
| 1 mL | 5 mg/mL | 0.2 mL · 20 units |
| 2 mL | 2.5 mg/mL | 0.4 mL · 40 units |
| 2.5 mL | 2 mg/mL | 0.5 mL · 50 units |
| 5 mL | 1 mg/mL | 1 mL · 100 units |
Illustrative arithmetic only, not a dose: concentration = 5 mg ÷ water; units = draw × 100 on a U-100 syringe. Your own compound, vial size, and dose go in the calculator.
The trade-off in one line
More water
Lower concentration, larger draw, more units on the barrel — easier to read and measure precisely. The limit is practical: the draw shouldn't exceed your syringe, and a bigger vial takes up more fridge space.
Less water
Higher concentration, tiny draw, few units — harder to measure without error, and very small volumes can leave the powder undissolved. If a dose works out to only a couple of units, that's the sign to add more water.
A simple way to pick
Rather than starting from the water, start from the draw you want to read. Decide a round, comfortable number of units per dose, and let that fix the water volume — the calculator's “units first” mode does exactly this. Whichever number you choose, use bacteriostatic water for a vial you'll draw from more than once.[1]
More in this guide
Part of the complete guide to reconstituting peptides.
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