Divide the milligrams of material in the vial by the concentration you want, in milligrams per millilitre. The answer is how many millilitres of bacteriostatic water to add. A 10 mg vial at a target of 5 mg/mL needs 2 mL.
That is the entire calculation. Everything below is either a shortcut that saves you from doing it, or a warning about the three ways people get it wrong. If you would rather not do arithmetic at all, the bac water calculator handles both directions.
The two forms of one formula
Concentration describes how much material sits in each millilitre of liquid. There is one relationship, and you use whichever form matches what you know:
Water to add (mL) = mg in vial ÷ target mg/mL
Concentration (mg/mL) = mg in vial ÷ mL added
If a vial holds 10 mg of lyophilized (freeze-dried) powder and you add 2 mL of bacteriostatic water, every millilitre now contains 5 mg. Add 5 mL instead and every millilitre contains 2 mg. The powder did not change. Only its distribution did.
Worked examples
| Material in vial | Water added | Concentration | Same value in mcg/mL |
|---|---|---|---|
| 5 mg | 1 mL | 5 mg/mL | 5,000 mcg/mL |
| 5 mg | 2 mL | 2.5 mg/mL | 2,500 mcg/mL |
| 5 mg | 5 mL | 1 mg/mL | 1,000 mcg/mL |
| 10 mg | 1 mL | 10 mg/mL | 10,000 mcg/mL |
| 10 mg | 2 mL | 5 mg/mL | 5,000 mcg/mL |
| 10 mg | 5 mL | 2 mg/mL | 2,000 mcg/mL |
| 20 mg | 4 mL | 5 mg/mL | 5,000 mcg/mL |
| 50 mg | 5 mL | 10 mg/mL | 10,000 mcg/mL |
Notice the pattern in any row pair: double the water, halve the concentration. Halve the water, double it.
Going the other way
When you know the concentration you want, divide into it instead:
| Material in vial | Target concentration | Water to add |
|---|---|---|
| 5 mg | 2.5 mg/mL | 2 mL |
| 10 mg | 5 mg/mL | 2 mL |
| 10 mg | 2 mg/mL | 5 mL |
| 15 mg | 5 mg/mL | 3 mL |
| 30 mg | 10 mg/mL | 3 mL |
Milligrams, micrograms and the factor of 1,000
Documentation flips between milligrams (mg) and micrograms (mcg or µg) constantly, and that is where most errors come from. The conversion never changes:
1 mg = 1,000 mcg
So 5 mg/mL is 5,000 mcg/mL, and 0.25 mg/mL is 250 mcg/mL. If a calculation produces something that looks a thousand times too large or too small, you have almost certainly mixed units rather than made an arithmetic mistake.
How to choose a volume
There is no universally correct amount of water. The choice is a trade-off between precision and space:
| More water (more dilute) | Less water (more concentrated) | |
|---|---|---|
| Measuring small volumes | Easier, because errors matter less | Harder, small errors have more effect |
| Total volume in vial | Larger, may not fit | Compact |
| Bac water consumed | More per preparation | Less |
Two practical limits to check before committing:
- Vial capacity. A small vial may not physically hold the volume your target concentration implies, and you want headspace left for mixing.
- Your bac water supply. A 10 mL vial of bacteriostatic water holds a finite number of preparations. The calculator’s third tool works out how many vials a project needs.
Adding the water, step by step
- Bring both vials to room temperature. Cold glass and condensation make everything harder.
- Swab both stoppers with 70% isopropyl alcohol and let them air-dry completely. Alcohol does much of its work as it evaporates.
- Draw your measured volume of bacteriostatic water. Technique matters here: see drawing from a multi-dose vial without coring the stopper.
- Run the water down the inside wall of the receiving vial rather than spraying it directly onto the powder.
- Swirl gently until the solid dissolves. Do not shake hard: vigorous shaking foams the solution and can damage delicate materials.
- Label immediately with the concentration and the date you prepared it.
Three mistakes that cost people a vial
1. Counting the powder as volume
For the small quantities in most vials, the solid’s own volume is negligible. Base the math on the water you added, not on the final visible volume.
2. Rounding halfway through
Carry full numbers through the calculation and round only the final answer. Rounding at each step compounds error.
3. Forgetting the water’s own clock
An opened vial of bacteriostatic water is good for 28 days. If your project runs longer than that, plan on opening a fresh vial partway through rather than stretching one. See how long bacteriostatic water lasts.
Why bacteriostatic water rather than something else?
Tap and distilled water are not sterile. Plain sterile water is, but it has no preservative, so once opened the remainder should be discarded. Bacteriostatic water contains 0.9% benzyl alcohol, which stops bacteria multiplying between withdrawals, and that is what lets one vial serve many preparations over several weeks. The comparison in full: bacteriostatic water vs sterile water.
Quick recap
- Water to add (mL) = mg in vial ÷ target mg/mL
- Concentration (mg/mL) = mg in vial ÷ mL added
- 1 mg = 1,000 mcg
- Check the vial can physically hold the volume
- Label with concentration and date
This guide covers laboratory preparation math. It is not medical or dosing advice. Run your numbers in the calculator, or read the full bacteriostatic water guide.
Frequently asked questions
How do you calculate how much bacteriostatic water to add?
Divide the milligrams of material in the vial by the concentration you want in mg per mL. For example, a 10 mg vial at a target of 5 mg/mL needs 2 mL of bacteriostatic water.
How do you work out the concentration after mixing?
Divide the milligrams in the vial by the millilitres of water you added. A 5 mg vial with 2 mL added is 2.5 mg/mL.
How many mcg are in 1 mg?
1,000. So 2.5 mg/mL is the same as 2,500 mcg/mL.
Does adding more bacteriostatic water reduce the amount of material?
No. The amount of material in the vial never changes. Adding more water only lowers the concentration, which makes small volumes easier to measure accurately.
How much water can a vial hold?
Check the vial’s capacity before you plan a volume. A small vial may not physically hold the amount of water your target concentration calls for, and you need headspace to mix.
Should I use bacteriostatic water or sterile water to reconstitute?
Bacteriostatic water if you will draw from the water vial more than once, because of its preservative. Sterile water is fine for a single preparation used immediately.
Sources
- Bacteriostatic Water for Injection, USP prescribing label, DailyMed, U.S. National Library of Medicine dailymed.nlm.nih.gov
- USP General Chapter <797> Pharmaceutical Compounding: Sterile Preparations usp.org
We write from primary sources such as FDA drug labels and USP standards. See our editorial policy. Spot an error? Email [email protected].
From BacWater Scientific
10 mL bacteriostatic water, stocked in the US
0.9% benzyl alcohol, multi-dose vials. The per-vial price drops automatically at 3, 5 and 10 vials.