How the amount of water you add sets the strength, with worked numbers.

Peptides are shipped as a dry powder. Adding water makes a solution whose concentration is simply vial mg ÷ mL of water:

• 10 mg + 2 mL = 5 mg/mL = 500 mcg per 10 units on a U-100 syringe.
• 10 mg + 1 mL = 10 mg/mL = 1,000 mcg per 10 units.
• 10 mg + 3 mL ≈ 3.33 mg/mL ≈ 333 mcg per 10 units.

The water doesn't change how much peptide is in the vial, only how spread out it is. More water makes small amounts easier to measure accurately; less water means smaller volumes. To find the volume for an amount: units = amount (mg) ÷ concentration (mg/mL) × 100.

Bacteriostatic water contains 0.9% benzyl alcohol as a preservative, which is why it's used for vials that will be entered more than once; its labels warn against use in newborns. The calculator and mixing guides do this arithmetic for you. They don't recommend any amount.

Quick check

Reconstitution calculator · Mixing charts for every vial size

Sources

• DailyMed: bacteriostatic water labels

General education, not medical advice. Nothing here recommends using any compound or amount.