Total Body Water Calculation with Intravascular and Extravascular Volumes
Use 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
Recommended Calculation Methods
Watson Formula (Preferred for Adults)
The Watson formula is the guideline-recommended method for estimating TBW in adults, derived from isotopic dilution techniques in populations including obese subjects 1:
- Males: TBW = 2.447 - (0.09156 × age) + (0.1074 × height in cm) + (0.3362 × weight in kg) 1
- Females: TBW = -2.097 + (0.1069 × height in cm) + (0.2466 × weight in kg) 1
Hume Formula (Alternative for Adults)
The Hume formula provides similar estimates to Watson and is equally acceptable 1:
- Males: TBW = (0.194786 × height) + (0.296785 × weight) - 14.012934 1
- Females: TBW = (0.34454 × height) + (0.183809 × weight) - 35.270121 1
Mellits-Cheek Formula (For Pediatric Patients)
Use the Mellits-Cheek formula for patients aged 1 month to 34 years (males) or 1 month to 31 years (females), derived from deuterium oxide distribution measurements 1
Understanding Body Water Distribution
Total Body Water Composition
TBW represents the entire water content of the body and serves as the volume of distribution for urea and other solutes 1. While the question asks about intravascular (IV) and extravascular (EV) volumes specifically, the anthropometric formulas calculate total body water as a single compartment 1.
- Intravascular volume (plasma volume) comprises approximately 7-8% of TBW
- Extravascular volume includes both interstitial fluid (approximately 25% of TBW) and intracellular fluid (approximately 65-70% of TBW)
Critical Pitfalls and Corrections
Fixed Fraction Method - DO NOT USE
Avoid using fixed fractions of body weight (0.60 for males, 0.55 for females, or 0.58 for all subjects) as this method is inaccurate and overestimates TBW even in overhydrated patients 1.
Edema Correction
For patients with edema, add the entire weight gain from fluid overload to the TBW calculated at dry weight 1:
- Calculate dry weight by subtracting edema weight from actual weight
- Apply Watson or Hume formula using dry weight
- Add total edema weight gain to the calculated TBW 1
This correction is necessary because both Watson and Hume formulas consistently underestimate TBW in overhydrated patients 1.
Amputation Corrections
For amputees, correct the anthropometric calculation to restore the proper relationship between body water content and body composition 1:
Three-step correction process:
- Calculate fraction of weight lost (fw) from amputation using standardized tables (e.g., leg below knee = 6.5%, arm at shoulder = 6.6%) 1
- Calculate hypothetical non-amputated weight = actual weight/(1 - fw), then apply Watson formula to get TBW at non-amputated weight 1
- Calculate actual TBW = actual weight × (TBW non-amputated / Weight non-amputated) 1
Extreme Body Habitus
Both Watson and Hume formulas provide unrealistic estimates in subjects whose height and/or weight differ greatly from ordinary ranges 1. In such cases, consider bioelectrical impedance analysis if available 2, 3.
Special Clinical Situations
Malnourished Patients
For severely malnourished patients, calculate TBW using their desired/target weight rather than actual weight when determining dialysis adequacy 1. This prevents the false appearance of adequate treatment that occurs when low body weight artificially elevates clearance ratios 1.
Obesity
The Watson and Hume formulas account for obesity because they were derived in populations including obese subjects 1. However, recognize that as obesity develops, TBW increases exponentially while body surface area increases linearly 1.
Accuracy Considerations
The Watson and Hume formulas provide estimates that are generally close to isotopic body water measurements in peritoneal dialysis patients 1. However, systematic underestimation occurs in both lean and obese patients with fluid overload 1, making the edema correction essential in these populations.
Anthropometric equations cannot be used to compare different populations (e.g., healthy vs. diseased), only to assess individual differences within a specific population 2. For comparing populations or when high precision is required, isotopic dilution techniques remain the gold standard 2, 4.