Total Body Water Calculation with Intravascular and Extravascular Volumes

Recommended Calculation Methods

The American Journal of Kidney Diseases recommends using the Watson or Hume anthropometric formulas to calculate total body water (TBW) in adults, and the Mellits-Cheek formula in children, based on sex, age, height, and actual body weight 1, 2, 3
The Watson formula is the guideline-recommended method for estimating TBW in adults, derived from isotopic dilution techniques in populations including obese subjects 1, 2
For males, TBW can be calculated using the Watson formula: TBW = 2.447 - (0.09156 × age) + (0.1074 × height in cm) + (0.3362 × weight in kg) 4
For females, TBW can be calculated using the Watson formula: TBW = -2.097 + (0.1069 × height in cm) + (0.2466 × weight in kg) 4
The Hume formula provides similar estimates to Watson and is equally acceptable for adults 1, 2
For males, TBW can be calculated using the Hume formula: TBW = (0.194786 × height) + (0.296785 × weight) - 14.012934 4
For females, TBW can be calculated using the Hume formula: TBW = (0.34454 × height) + (0.183809 × weight) - 35.270121 4

Total body water (TBW) represents the entire water content of the body and serves as the volume of distribution for urea and other solutes 1
Intravascular volume (plasma volume) comprises approximately 7-8% of TBW 4
Extravascular volume includes both interstitial fluid (approximately 25% of TBW) and intracellular fluid (approximately 65-70% of TBW) 4

Avoid using fixed fractions of body weight to estimate TBW, as this method is inaccurate and overestimates TBW even in overhydrated patients 1, 2, 3
For patients with edema, add the entire weight gain from fluid overload to the TBW calculated at dry weight 1, 2
Both Watson and Hume formulas consistently underestimate TBW in overhydrated patients 1, 2, 3
For amputees, correct the anthropometric calculation to restore the proper relationship between body water content and body composition 6, 7, 8
For severely malnourished patients, calculate TBW using their desired/target weight rather than actual weight when determining dialysis adequacy 1, 3
The Watson and Hume formulas account for obesity because they were derived in populations including obese subjects 1, 2, 3

The Watson and Hume formulas provide estimates that are generally close to isotopic body water measurements in peritoneal dialysis patients 1, 2
However, systematic underestimation occurs in both lean and obese patients with fluid overload 1, 2, 3