Bacteriostatic water calculator for peptide vials

The amount of bacteriostatic water you add decides the concentration, so it can be worked backwards from the units you want each dose to be. Water (mL) = units per dose ÷ 100 × vial strength ÷ dose. For a 5 mg vial and a 250 mcg dose, 2 mL of water makes each dose exactly 10 units on a U-100 syringe.

The formula

Doses per vial: vial strength ÷ dose
Volume per dose: units you want to draw ÷ 100 = mL
Water to add: doses per vial × volume per dose = mL
Concentration: vial strength ÷ water added = mg per mL

Worked example

A 10 mg vial and a 500 mcg dose: 10,000 ÷ 500 = 20 doses in the vial.
You want each dose to be 10 units, which is 0.1 mL.
20 doses × 0.1 mL = 2 mL of water to add.
Check: 10 mg ÷ 2 mL = 5 mg/mL, and 0.5 mg ÷ 5 mg/mL = 0.1 mL = 10 units.

Bac water chart

Water to add so that each dose is 10 units on a U-100 syringe
Vial	100 mcg dose	250 mcg dose	500 mcg dose	1 mg dose
2 mg	2 mL	0.8 mL	0.4 mL	0.2 mL
5 mg	5 mL	2 mL	1 mL	0.5 mL
10 mg	10 mL	4 mL	2 mL	1 mL
15 mg	15 mL	6 mL	3 mL	1.5 mL

Charts by vial size

Common questions

How much bac water do I add to a 5 mg vial?

It depends on the dose and on how many units you want each dose to be. For a 250 mcg dose drawn as 10 units, add 2 mL. For the same dose drawn as 5 units, add 1 mL. Use the volume on the product label or from your pharmacist if one is given.

How much bac water for a 10 mg vial?

For a 500 mcg dose drawn as 10 units, add 2 mL, which gives 5 mg/mL. For a 250 mcg dose drawn as 10 units it would be 4 mL, which may not fit the vial, so 2 mL with each dose drawn as 5 units is the same arithmetic in a smaller volume.

Does more water make the peptide weaker?

It lowers the concentration, so each unit on the syringe holds less. The total amount in the vial and the number of doses stay the same.

What if the result is more water than the vial holds?

Choose a smaller number of units per dose. Halving the units halves the water needed. Small vials commonly hold about 2 to 3 mL, but check yours.

Is bacteriostatic water the same as sterile water?

No. Bacteriostatic water contains a preservative, usually 0.9% benzyl alcohol, and sterile water does not. Which one to use is stated on the product label or by your pharmacist.

Other calculators

Keep the numbers with your log

Record each vial's strength and status in PepVault, next to every dose you log. Free for Android, iPhone, iPad and Mac.

For Android, iPhone, iPad and Mac. On the App Store the app is listed as PeptideMate.