“Fastest” depends on two scenarios: (1) increasing daily intake through diet, or (2) treating medically low blood calcium (hypocalcemia). For diet, the fastest safe approach is to anchor a high-calcium item into daily routine: yogurt/kefir, milk, calcium-fortified beverages, or calcium-set tofu. “Fast increase via supplements” should be clinician-guided; unnecessary high doses can cause constipation, raise kidney-stone risk, and create other issues in susceptible people. Calcium effectiveness is also linked to vitamin D—if vitamin D is low, absorption drops and results may disappoint. For supplements, splitting doses (avoiding very large single doses, e.g., >500–600 mg at once) can improve absorption. If the goal is correcting low serum calcium, do not self-treat aggressively; the cause (parathyroid, kidney function, vitamin D) should be identified first and managed accordingly.