How to Use This Calculator
Choose a mode first. If you have already mixed the vial, use “I know the water I added”: enter the vial size, the water you added, the dose in mcg or mg and the syringe size, then click Calculate to see the units and mL to draw, the concentration and how many doses the vial holds. If you have not mixed it yet and want a set strength, use “I want a target concentration”: enter the vial size and the concentration you want, and the calculator tells you how much water to add.
Common Vial and Water Combinations
The table shows the concentration you get from common vial and water amounts, and how many units a few doses take on a U-100 syringe. It is only arithmetic, not a dosing guide.
| Vial | BAC water | Concentration | 100 mcg | 250 mcg | 500 mcg | 1 mg |
|---|---|---|---|---|---|---|
| 2 mg | 1 mL | 2 mg/mL | 5 units | 12.5 units | 25 units | 50 units |
| 2 mg | 2 mL | 1 mg/mL | 10 units | 25 units | 50 units | 100 units |
| 5 mg | 1 mL | 5 mg/mL | 2 units | 5 units | 10 units | 20 units |
| 5 mg | 2 mL | 2.5 mg/mL | 4 units | 10 units | 20 units | 40 units |
| 5 mg | 2.5 mL | 2 mg/mL | 5 units | 12.5 units | 25 units | 50 units |
| 5 mg | 3 mL | 1.67 mg/mL | 6 units | 15 units | 30 units | 60 units |
| 10 mg | 1 mL | 10 mg/mL | 1 unit | 2.5 units | 5 units | 10 units |
| 10 mg | 2 mL | 5 mg/mL | 2 units | 5 units | 10 units | 20 units |
| 10 mg | 3 mL | 3.33 mg/mL | 3 units | 7.5 units | 15 units | 30 units |
| 10 mg | 5 mL | 2 mg/mL | 5 units | 12.5 units | 25 units | 50 units |
How Much Water Should You Add?
There is no single right amount. Less water makes a stronger solution, so each dose is a smaller draw. More water makes a weaker solution, so each dose is a bigger draw that is easier to read. A good rule is to pick a volume that puts your usual dose somewhere between about 10 and 50 units, where the syringe markings are easy to line up. Very small draws, under about 5 units, are hard to measure accurately. The Peptide Reconstitution Volume Optimizer compares every common water volume for the dose, and the Syringe Volume Calculator shows where a draw sits on each barrel size.For the full mixing steps, see our guide on how to reconstitute a peptide.
Bacteriostatic Water or Sterile Water?
Bacteriostatic water is sterile water with 0.9% benzyl alcohol added. The benzyl alcohol slows bacterial growth, which is why it is used for vials that will be drawn from more than once. Plain sterile water has no preservative, so it suits single-use vials. Our guide on BAC water vs sterile water explains the difference in more detail, and the Peptide Stability and Storage Estimator shows how storage affects a mixed vial.
Who Should Use This
Researchers who need to reconstitute a lyophilised peptide vial and turn a dose in mcg or mg into syringe units, or who want to mix vials to a set concentration.
What This Calculator Does
The Bac Water Calculator, also called a peptide reconstitution calculator or bacteriostatic water calculator, works in two directions. From a vial size, the bacteriostatic water added and a dose, it gives the units and mL to draw on a U-100 syringe, the concentration and the number of doses in the vial. From a vial size and a target concentration, it gives the amount of water to add.
Formula Used
Concentration (mg/mL) = Vial size (mg) / BAC water (mL)Volume per dose (mL) = Dose (mg) / ConcentrationUnits to draw = Volume (mL) x 100 (U-100 syringe)Doses per vial = FLOOR(Vial size / Dose)BAC water to add (mL) = Vial size (mg) x 1,000 / Target concentration (mcg/mL)Example Calculation
Limitations
This is a unit-conversion tool, not dosing guidance. It assumes the whole vial dissolves, the label amount is exact and a U-100 syringe is used, and it ignores dead space and draw loss.
Frequently Asked Questions
How many units is 250 mcg in a 5 mg vial?
It depends on the water added. With 2 mL, 250 mcg is 10 units. With 2.5 mL, it is 12.5 units. With 5 mL, it is 25 units. All three are on a U-100 syringe.
How much bacteriostatic water should I add to a 5 mg vial?
Choose the amount that makes your usual dose easy to measure. 2 mL gives 2.5 mg/mL and 2.5 mL gives 2 mg/mL, and both are common. If the dose comes out under about 5 units, add more water so the draw is bigger.
How many units is 1 mg in a 10 mg vial?
With 1 mL of water, 1 mg is 10 units. With 2 mL, it is 20 units. With 3 mL, it is 30 units. The more water you add, the more units each milligram takes.
Does adding more water change the dose?
No. The amount of peptide in the vial stays the same. More water only spreads it through more liquid, so you draw more units to get the same dose.
What is the difference between mg and mcg?
1 mg equals 1,000 mcg. Vials are labelled in mg, while many doses are written in mcg, so a 5 mg vial holds 5,000 mcg. The calculator lets you enter the dose in either unit.
How many units is 2 mL or 3 mL of bac water?
2 mL is 200 units and 3 mL is 300 units on a U-100 insulin syringe, because 100 units equals 1 mL. A standard 1 mL syringe holds 100 units, so these volumes take more than one fill or a larger syringe.
Do you need bac water for peptides?
Bacteriostatic water is sterile water with 0.9% benzyl alcohol added, which slows bacterial growth. That is why it is used for vials that are drawn from more than once. Plain sterile water has no preservative, so it suits single-use vials.