Volume = Mass of Powder / Desired Concentration
Reconstitution is the process of adding a diluent (water, saline, buffer) to a lyophilized (freeze-dried) powder to create a solution for injection or use. The volume of diluent = (mass of powder / desired concentration) − powder displacement volume. Powder displacement accounts for the volume occupied by the solid powder itself. Example: 500 mg powder with 0.2 mL displacement, target 50 mg/mL: final volume = 10 mL, diluent = 10 − 0.2 = 9.8 mL. Reconstitution is standard practice in pharmacy for antibiotics (e.g., ceftriaxone, vancomycin), biologics, and vaccines. Always use the manufacturer-specified diluent, reconstitute gently (do not shake proteins), and note the beyond-use date after reconstitution.