The sum of the ICW and the ECW – the total amount of water contained within the body.
The total body water is the sum total of the fluid volume of the patient. TBW is further subdivided into Intra-Cellular Water (ICW) and Extra-Cellular Water (ECW). TBW is dependent upon the hydration status of the patient as well as body composition. Lean tissue is approximately 73 percent water whereas fat tissue is approximately 10 percent water.
Consider these quotes from the medical textbook Fluid, Electrolyte and Acid-Base Balance: (Horne, Heinz, & Swearingen; Mosby, 1991.
“Water is the primary constituent of the human body. All body fluids are dilute solutions of water and dissolved substances (solutes). The average 70 kg adult male is approximately 60% water by weight, while the average female is approximately 55% water by weight.” “The percentage of body weight that is water varies with such factors as age, gender, and body fat content.”