Reconstitution maths tells you the concentration a diluent produces, but not which diluent to reach for. Bacteriostatic water and sterile water are the two most commonly referenced options in a peptide-handling context, and the difference between them comes down to one added ingredient.
What separates the two
Sterile water is exactly what it sounds like — water that's been sterilised and contains nothing else. Bacteriostatic water is the same starting point with a small amount of benzyl alcohol added, typically around 0.9%, as a preservative.
That one addition is the entire practical difference. Both are sterile at the point of use; only one contains an ingredient whose job is to inhibit microbial growth in the solution afterwards.
Why the preservative matters for a stored solution
Once a lyophilized peptide is dissolved, it becomes a far more hospitable environment for microbial growth than the dry powder was. A preservative doesn't change how well the peptide dissolves or the resulting concentration, but it does slow contamination if the same reconstituted vial is going to be accessed more than once over a period of days.
Sterile water has no such protection built in — a solution prepared with it is generally treated as a single-session preparation, evaluated and used promptly rather than stored and re-accessed repeatedly.
What this doesn't change
Neither diluent choice affects the reconstitution arithmetic itself — concentration is still peptide mass divided by diluent volume, regardless of which water is used. The choice is a stability and handling-window question, not a concentration one, and doesn't substitute for refrigeration or the other cold-chain practices covered in our storage guidance.