Bacteriostatic water reconstitution calculator
Enter how much powder is in the vial and how much bacteriostatic water you are adding, and this works out the concentration: how much substance ends up in every millilitre, and how much sits in each unit mark of a 1 mL U-100 insulin syringe.
It is free, it shows its working, and it is the same arithmetic printed on every reconstitution chart - done for you, with the syringe conversion included.
The calculator
5 mg/mL
- 0.05 mg in 1 unit mark (1 mL U-100 insulin syringe)
- 0.5 mg in 10 unit marks
- 200 units in the whole vial
10 mg ÷ 2 mL = 5 mg/mL
5 mg/mL ÷ 100 unit marks per mL = 0.05 mg per unit mark
A value written with a leading “~” has been rounded for display. International Units and milligrams are never converted into each other: an International Unit is defined per substance, so there is no general conversion factor. Use whichever unit is printed on your vial.
The formula, published
There are only two steps, and both are division.
concentration = powder amount / water volume
Powder in milligrams divided by water in millilitres gives milligrams per millilitre. Powder in International Units divided by millilitres gives IU per millilitre. The two unit systems never mix.
amount per unit mark = concentration / unit marks per mL
A U-100 insulin syringe carries 100 unit marks to the millilitre, so one mark is 0.01 mL. Dividing the concentration by 100 gives what a single mark holds.
To go the other way - from a volume in the syringe back to an amount - multiply:
amount = concentration × volume drawn
The volume of the powder itself is not counted. At these amounts it displaces far less liquid than the smallest line on the syringe can resolve, which is the same assumption every printed reconstitution chart makes.
Worked example
A 10 mg vial mixed with 2 mL of bacteriostatic water:
- 10 mg ÷ 2 mL = 5 mg/mL
- 5 mg/mL ÷ 100 unit marks per mL = 0.05 mg per unit mark
- 5 mg/mL × 0.4 mL = 2 mg (0.4 mL is 40 units)
So the mixed vial is 5 mg/mL. Each unit mark on the syringe holds 0.05 mg, ten marks hold 0.5 mg, and the whole vial is 200 units.
Every compound, worked out in full
25 pages, one for each vial size. Every one shows the full chart of water volumes, the arithmetic, and what each syringe mark holds - no JavaScript needed.
How much bacteriostatic water do you need?
You need about 10 mL of bacteriostatic water to cover any single mix on this site.
Every water volume in these charts is between 0.5 mL and 10 mL, so a single 10 mL vial of bacteriostatic water covers the largest of them exactly. Bacteriostatic water is sterile water that contains 0.9% benzyl alcohol as a preservative, which is what allows the same vial to be punctured more than once.
Storage and shelf life
Bacteriostatic water is sterile water that contains 0.9% benzyl alcohol as a preservative. The preservative is the reason a vial can be entered more than once rather than being discarded after a single use. An unopened vial keeps until the expiry date printed on it; once the stopper has been punctured, the discard window printed in the manufacturer’s insert for that vial applies.
Store water and mixed vials as the labelling for each one directs, out of direct light. If the liquid changes colour, goes cloudy, or shows anything floating in it, it goes in the bin rather than in a syringe.
Common questions
- What does this calculator actually calculate?
- Concentration. It divides the amount of powder in a vial by the volume of bacteriostatic water added to it, and then converts that into the amount sitting in each unit mark of a U-100 insulin syringe. Nothing else.
- Does it tell you what amount to use?
- No, and it never will. This is dilution arithmetic. What to use, how often, and whether to use anything at all are questions for the prescriber or the researcher responsible for the vial. A calculator has no business answering them.
- Why are the answers given in syringe unit marks as well as millilitres?
- Because unit marks are what people can actually read off a barrel. A U-100 insulin syringe is marked in 100 units to the millilitre, so one unit mark is 0.01 mL. Reading 0.23 mL off a barrel is guesswork; reading the 23 mark is not.
- Can it convert International Units into milligrams?
- No, and refusing is the correct answer rather than a missing feature. An International Unit is defined separately for each substance against a biological reference standard, so there is no general conversion factor. Use whichever unit is printed on your vial; the arithmetic is identical either way.
- How much bacteriostatic water does a single mix use?
- Across every compound and vial size on this site the water volumes run from 0.5 mL to 10 mL. A 10 mL vial of bacteriostatic water therefore covers the great majority of single mixes, and a 30 mL vial covers repeated mixing.
Brand names that appear on these pages are registered trademarks of their respective owners. This site is not affiliated with, endorsed by or sponsored by any of them, and supplies bacteriostatic water only - never the compounds the water is used to mix.