What the water actually changes.
Bacteriostatic water sets the concentration and nothing else. The milligrams in the vial are fixed by the label, so the volume you add decides only how many units a given amount reads.
The direct answer
The water volume divides into the vial mass to give the concentration. More water makes a weaker solution, so the same amount is drawn to a higher unit mark. It never changes how much peptide the vial contains.
mg/mL = vial mg ÷ water mL
10 mg in 2 mL gives 5 mg/mL, so 10 units on a U-100 syringe contains 0.5 mg. The same vial in 5 mL gives 2 mg/mL, and 10 units then contains 0.2 mg.
This page is arithmetic only. It converts numbers you already have from the vial in front of you and the prescription you were given. It does not tell you how much water to add, how much to take, or how often. Those are medical decisions for a licensed clinician who knows your history.
What each pairing produces
Vial mass down the side, water across the top. Each cell is the resulting concentration and what a single U-100 unit then holds.
| Vial | 1 mL | 2 mL | 3 mL | 5 mL |
|---|---|---|---|---|
| 1 mg | 1 mg/mL0.01 mg per unit | 0.5 mg/mL0.005 mg per unit | 0.333 mg/mL0.003 mg per unit | 0.2 mg/mL0.002 mg per unit |
| 5 mg | 5 mg/mL0.05 mg per unit | 2.5 mg/mL0.025 mg per unit | 1.667 mg/mL0.017 mg per unit | 1 mg/mL0.01 mg per unit |
| 10 mg | 10 mg/mL0.1 mg per unit | 5 mg/mL0.05 mg per unit | 3.333 mg/mL0.033 mg per unit | 2 mg/mL0.02 mg per unit |
| 17 mg | 17 mg/mL0.17 mg per unit | 8.5 mg/mL0.085 mg per unit | 5.667 mg/mL0.057 mg per unit | 3.4 mg/mL0.034 mg per unit |
| 18 mg | 18 mg/mL0.18 mg per unit | 9 mg/mL0.09 mg per unit | 6 mg/mL0.06 mg per unit | 3.6 mg/mL0.036 mg per unit |
| 30 mg | 30 mg/mL0.3 mg per unit | 15 mg/mL0.15 mg per unit | 10 mg/mL0.1 mg per unit | 6 mg/mL0.06 mg per unit |
| 70 mg | 70 mg/mL0.7 mg per unit | 35 mg/mL0.35 mg per unit | 23.333 mg/mL0.233 mg per unit | 14 mg/mL0.14 mg per unit |
Forty units, drawn
Questions people actually type
How much bacteriostatic water should I add to a vial?
That is a prescription question, not an arithmetic one, and this site does not answer it. What the arithmetic can tell you is the consequence of a volume you have already been given: the water you add divides into the vial mass to set the concentration, and a larger volume makes a weaker solution that reads as more units for the same mass.
Does adding more water change how much peptide is in the vial?
No. The mass in the vial is fixed at whatever the label says. Water changes only the concentration and therefore the reading on the syringe. A 10 mg vial holds 10 mg whether you add 1 mL or 5 mL, but at 10 mg/mL a 1 mg amount reads 10 units, and at 2 mg/mL the same 1 mg reads 50 units.
What concentration does a 10 mg vial with 2 mL give?
Five milligrams per millilitre. Ten divided by two is five. On a U-100 syringe, where one unit is 0.01 mL, each unit of that solution holds 0.05 mg, so 10 units contains 0.5 mg and 40 units contains 2 mg.
Why do two people with the same vial get different unit numbers?
Because they added different amounts of water. The vial mass is the same, so the concentration differs, and the syringe reading follows the concentration. This is the single most common reason two sets of numbers for the same compound do not agree.
Related conversions
Worked vial cases
Run your own numbers
The free web calculator takes your vial, your water, and your amount, and shows every step of the conversion. Nothing is stored and no account is needed.