Bacteriostatic Water Calculator
How much bacteriostatic water to add to a vial so each dose lands on an easy line, like 10 or 20 units, on a U-100 syringe.
Updated · By Niraj Kumar · 3 sources
https://checkpre.com/health/bacteriostatic-water-calculator
Medication
Vial strength
The total mg on the vial.
10 or 20 units are easy to read on any syringe.
Your dose
Exactly as written on your prescription. 1 mg is 1,000 mcg.
- Dose
- 0.25 mg
- 250 mcg
- Concentration
- 2.5 mg/mL
- Exact volume
- 0.1 mL
- 10 units (1 unit = 0.01 mL on a U-100 syringe)
- Doses in the vial
- 20
- 20 weeks at one dose a week. Most multi-dose vials must be discarded 28 days after first use; check yours.
- Check
- 5 mg ÷ 2 mL
- 0.25 mg ÷ 2.5 mg/mL × 100 = 10 units (you asked for 10 units)
| Weekly dose | When | Units | mL |
|---|---|---|---|
| 0.25 mg | Weeks 1–4 | 10 units | 0.1 mL |
| 0.5 mg | Weeks 5–8 | 20 units | 0.2 mL |
| 1 mg | Weeks 9–12 | 40 units (over capacity) | 0.4 mL |
| 1.7 mg | Weeks 13–16 | 68 units (over capacity) | 0.68 mL |
| 2.4 mg | Week 17 on (usual maintenance) | 96 units (over capacity) | 0.96 mL |
| 7.2 mg | Only if prescribed after 4+ weeks at 2.4 mg | 288 units (over capacity) | 2.88 mL |
How much bacteriostatic water should you add?
Water (mL) = vial mg × units you want per dose ÷ (dose mg × 100). To make a 0.25 mg dose draw 10 units from a 5 mg vial, add 5 × 10 ÷ 25 = 2 mL, which gives 2.5 mg/mL. Check any change to how a prescribed vial is mixed with your pharmacy first.
units = dose (mg) ÷ concentration (mg/mL) × 100This page uses the same method as the GLP-1 Dosage Calculator, which explains each term and assumption in full.
| Water added | Concentration | Units per dose |
|---|---|---|
| 0.5 mL | 20 mg/mL | 12.5 units |
| 1 mL | 10 mg/mL | 25 units |
| 2 mL | 5 mg/mL | 50 units |
| 3 mL | 3.333 mg/mL | 75 units |
Worked example
A 5 mg vial, a 0.25 mg dose, and doses wanted at 10 units each.
- 1Water: 5 mg × 10 units ÷ (0.25 mg × 100) = 2 mL
- 2Concentration: 5 mg ÷ 2 mL = 2.5 mg/mL
- 3Check: 0.25 ÷ 2.5 × 100 = 10 units
Add 2 mL of bacteriostatic water; each 0.25 mg dose is then 10 units.
Frequently asked questions
Why do pharmacies use bacteriostatic water?
It is sterile water with 0.9% benzyl alcohol, which stops bacteria growing between doses, so one vial can be used for several weeks. Plain sterile water is for single use. Use the diluent your pharmacy specifies.
Is there a limit to how much water I can add?
The vial has to hold it, and most hold 2 to 3 mL comfortably. More water also means more injected volume per dose. If the answer is over the vial's size, choose fewer units per dose.
Sources
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