The total body water (TBW) in a healthy adult typically constitutes about 60-65% of body weight, reflecting the large water content in muscle and other lean tissues. This percentage is crucial for maintaining cellular function, electrolyte balance, and blood volume. Understanding this proportion is essential in fluid management during surgery and critical care to prevent complications such as hypovolemia or fluid overload. Variations occur with age, sex, and body composition, but the 60-65% range remains the standard for healthy adults.

Reference: Ganong's Review of Medical Physiology, 26th Edition.