Bacteriostatic Water Calculator

What reconstitution actually decides

A lyophilised vial is a plug of dry powder with a milligram figure printed on the label. It has no concentration at all until you add a diluent, and the volume you choose is what sets it. Bacteriostatic Water for Injection, USP is the diluent normally specified: sterile water with 0.9% benzyl alcohol added as a preservative, supplied in a multiple-dose container so repeated withdrawals can be made from the same vial.

The arithmetic is one division. Concentration equals the milligrams in the vial divided by the millilitres of water added. A 5 mg vial reconstituted with 2 mL is 2.5 mg/mL; the same vial with 1 mL is 5 mg/mL. Nothing else about the vial changes.

That last point is the one people get wrong. Adding more water does not make the vial go further — it dilutes it. A 5 mg vial holds ten 0.5 mg doses whether you reconstitute it with 1 mL or with 5 mL. What the water volume changes is the size of the draw: the same 0.5 mg dose is 10 units on a U-100 syringe at 5 mg/mL, and 50 units at 1 mg/mL. Choosing a larger volume buys you a bigger, easier-to-read mark on the barrel, and costs you nothing except refrigerator space and the point at which the dose stops fitting the syringe.

A U-100 insulin syringe is graduated so that 100 units is 1 mL. That is the conversion that turns a concentration into something you can actually draw, and it is why a dose over 1 mL cannot be given on an insulin barrel at all.

Bacteriostatic water chart — concentration by vial size and water added

Resulting concentration in mg/mL. Read down for the milligrams printed on your vial, across for the water you are adding.

The lower figure is how much drug one unit mark on a U-100 insulin syringe delivers at that concentration. Every cell is computed by the same function the calculator above uses.

Water to add for a target concentration

Millilitres of bacteriostatic water needed to bring each vial to the concentration in the column heading.

“BnOH” is the benzyl alcohol carried by that volume of 0.9% bacteriostatic water, at 9 mg/mL. The largest vial in routine supply is 30 mL, and volumes much under 0.5 mL cannot be measured accurately enough to be worth aiming at — choose a lower concentration instead.

Units on a U-100 syringe, by dose and concentration

What one dose reads on an insulin barrel once the vial is reconstituted. Read down for your dose, across for the concentration you mixed.

The lower line is the smallest insulin barrel that holds the draw, because the smallest barrel that fits reads finest. Anything over 100 units is more than 1 mL and no insulin syringe holds it.

Doses per vial

Whole doses in each vial. Note that water volume does not appear — diluting further does not create more drug.

Part doses are not counted. How long the vial then lasts depends on your dosing frequency and on the beyond-use date of the reconstituted solution, which is set by the drug’s own label, not by the diluent.

Frequently asked questions

How much bacteriostatic water do I add to a vial?

As much as it takes to reach the concentration you want: water in mL equals the milligrams in the vial divided by your target mg/mL. A 5 mg vial with 2 mL of bacteriostatic water gives 2.5 mg/mL; with 1 mL it gives 5 mg/mL; with 5 mL it gives 1 mg/mL. There is no single correct volume — pick the one that puts your dose on an easy mark, then keep it consistent for that vial.

What is the difference between bacteriostatic water and sterile water?

Bacteriostatic Water for Injection, USP contains 0.9% (9 mg/mL) benzyl alcohol as a bacteriostatic preservative and is supplied in a multiple-dose container, so repeated withdrawals can be made from the same vial. Sterile Water for Injection contains no bacteriostat, antimicrobial agent or buffer and is supplied only in single-dose containers — once opened, the unused portion is discarded. If a vial is going to be entered more than once, the preservative is the whole point.

Can bacteriostatic water be used in babies or neonates?

No. The FDA label carries the warning "NOT FOR USE IN NEONATES" on the front of the vial. Benzyl alcohol has been associated with toxicity in neonates, and the contraindications section states that solutions containing benzyl alcohol must not be used in this population. Where water is required to prepare or dilute medication for a neonate, only preservative-free Sterile Water for Injection should be used. This is a hard contraindication, not a caution.

How long does a reconstituted vial last?

That is set by the drug you reconstituted, not by the water. The bacteriostatic water label explicitly says not to store reconstituted solutions unless the manufacturer of the solute directs otherwise. As a real example, the NOVAREL chorionic gonadotropin label states that when reconstituted with bacteriostatic water preserved with 0.9% benzyl alcohol, the solution should be refrigerated at 2–8°C and used within 30 days. Check your own product label for its figure.

How long is a bacteriostatic water vial good for once opened?

The preservative is what allows repeated entry, but it does not make the vial last indefinitely. CDC guidance, following USP General Chapter <797>, is that once a multiple-dose vial is punctured it should be dated and discarded within 28 days unless the manufacturer states another date, and the beyond-use date must never exceed the printed expiry. Write the date on the vial at first puncture.

How do I store bacteriostatic water?

Unopened, at 20–25°C (68–77°F), which is USP Controlled Room Temperature — the same cupboard as any other medication, away from light and heat. It does not need refrigerating. The reconstituted drug usually does: once powder is in solution, most labels call for 2–8°C. Swab the stopper with alcohol before every entry and do not use the vial if the solution is not clear or the seal is not intact.

How many units is my dose on an insulin syringe?

Multiply the dose volume in mL by 100, because a U-100 syringe is graduated so 100 units is 1 mL. At 2.5 mg/mL, a 0.5 mg dose is 0.2 mL, which is 20 units. At 5 mg/mL the same 0.5 mg dose is 10 units. A dose over 1 mL — anything past the 100-unit mark — does not fit an insulin barrel and needs either a stronger concentration or a split.

Does adding more water mean fewer doses in the vial?

No. The vial holds the same milligrams whatever you dilute it with, so a 5 mg vial gives ten 0.5 mg doses at 1 mL, at 2 mL and at 5 mL. What changes is the size of the draw: 10 units, 20 units and 50 units respectively for that same 0.5 mg. More water means a larger, easier-to-read mark, not a longer-lasting vial.

Is there a limit on how much bacteriostatic water is safe?

There is a labelled figure. Animal studies of small-volume parenterals containing 0.9% benzyl alcohol indicate an estimated intravenous dose of up to 30 mL may be given to an adult without toxic effects — that is 270 mg of benzyl alcohol. A typical 0.2 mL subcutaneous dose carries about 1.8 mg, so ordinary reconstitution volumes sit far below it. The label also states bacteriostatic water is for drug diluent use only and must not be used for fluid replacement.

Sources

1. [1]Bacteriostatic Water for Injection, USP — FDA prescribing information (Hospira, Inc.), DailyMed. Warning: NOT FOR USE IN NEONATES; 0.9% (9 mg/mL) or 1.1% (11 mg/mL) benzyl alcohol added as a bacteriostatic preservative; pH 5.7 (4.5 to 7.0); store at 20–25°C
2. [2]Sterile Water for Injection, USP — FDA prescribing information, DailyMed. "Contains no bacteriostat, antimicrobial agent or added buffer and is supplied only in single-dose containers"
3. [3]NOVAREL (chorionic gonadotropin) for injection — FDA prescribing information, DailyMed. Reconstitution table (10,000 IU vial: 1 mL gives 10,000 IU/mL, 10 mL gives 1,000 IU/mL) and 30-day refrigerated in-use period when reconstituted with 0.9% benzyl alcohol bacteriostatic water
4. [4]Centers for Disease Control and Prevention. Preventing Unsafe Injection Practices — multiple-dose vials should be dated and discarded within 28 days of first puncture unless the manufacturer states otherwise (USP General Chapter <797>)