Watson Formula (Male)
40yo male, 70kg, 175cm
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Evidence-based calculations Used in clinical settings worldwide Regular monitoring recommended
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Adult Male
40yo male, 70kg, 175cm
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Adult Female
35yo female, 60kg, 165cm
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Elderly Patient
75yo male, reduced TBW
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Obese Patient
120kg male - TBW considerations
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Athletic Female
28yo athlete with higher muscle mass
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Pediatric Patient
8yo child, 25kg
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Clinical Scenarios
Click a scenario to load values for different patient types:
Adult Male
40yo male, 70kg, 175cm
Standard adult male for baseline TBW assessment
Adult Female
35yo female, 60kg, 165cm
Standard adult female - note lower TBW percentage due to higher fat mass
Elderly Patient
75yo male, reduced TBW
Elderly male with age-related decrease in TBW percentage
Obese Patient
120kg male - TBW considerations
Morbidly obese patient - fat has lower water content, affects TBW%
Athletic Female
28yo athlete with higher muscle mass
Lean athlete - muscle has high water content, higher TBW%
Pediatric Patient
8yo child, 25kg
Child for TBW estimation using Mellits-Cheek formula
Patient Parameters
Calculation Settings
Total Body Water Results
42.0
Total Body Water (L)
60.0% of body weight
28.0
ICF (L)
2/3 of TBW
14.0
ECF (L)
1/3 of TBW
3.5
Plasma (L)
1/4 of ECF
10.5
Interstitial (L)
3/4 of ECF
Total body water: 42.0 L (60.0% body weight). ICF: 28.0 L, ECF: 14.0 L. Plasma volume: 3.5 L, Blood volume: ~6.4 L.
Clinical Applications
- Total Body Water: 42.0 L (60.0% body weight)
- Intracellular Fluid (ICF): 28.0 L (2/3 of TBW)
- Extracellular Fluid (ECF): 14.0 L (1/3 of TBW)
Fluid Distribution Visualization
Fluid Compartments
Formula Comparison
Body Composition
Step-by-Step Calculations
Step 1: Watson Formula (Male)
Formula: ext{TBW} = 2.447 - (0.09156 x ext{age}) + (0.1074 x ext{height}) + (0.3362 x ext{weight})
Calculation: TBW = 2.447 - (0.09156 x 40) + (0.1074 x 175) + (0.3362 x 70)
Result: 41.11 L
Step 2: Hume-Weyers Formula
Formula: TBW = (0.194786 x height) + (0.296785 x weight) - 14.012934
Calculation: TBW = (0.194786 x 175) + (0.296785 x 70) - 14.012934
Result: 40.85 L
Step 3: Simplified Formula
Formula: TBW = 0.6 x weight
Calculation: TBW = 0.6 x 70
Result: 42.00 L
Step 4: Calculate Intracellular Fluid (ICF)
Formula: ext{ICF} = ext{TBW} x (2/3)
Calculation: ICF = 42.00 x 0.667
Result: 28.00 L
Step 5: Calculate Extracellular Fluid (ECF)
Formula: ext{ECF} = ext{TBW} x (1/3)
Calculation: ECF = 42.00 x 0.333
Result: 14.00 L
Step 6: Calculate Plasma Volume
Formula: ext{Plasma} = ext{ECF} x 0.25
Calculation: Plasma = 14.00 x 0.25
Result: 3.50 L
TBW Reference Values
| Population | TBW as % Body Weight | TBW Factor | Notes |
| Newborn | 70-80% | 0.75 | Highest percentage of any age group |
| Infant (1 year) | 60-65% | 0.60-0.65 | Gradually decreases from birth |
| Adult Male | 55-65% | 0.60 | Higher muscle mass = higher TBW% |
| Adult Female | 45-55% | 0.50 | Higher fat mass = lower TBW% |
| Elderly Male | 50-55% | 0.50 | Decreased muscle, increased fat |
| Elderly Female | 45-50% | 0.45 | Further reduced TBW with age |
| Obese | 40-50% | 0.40-0.50 | Fat has low water content (~10%) |
Body Fluid Compartments
| Compartment | % of TBW | % of Body Weight | Key Contents |
| Intracellular Fluid (ICF) | 67% | 40% | K+, Mg2+, phosphate, proteins |
| Extracellular Fluid (ECF) | 33% | 20% | Na+, Cl-, HCO3- |
| - Interstitial Fluid | 25% | 15% | Between cells, similar to plasma |
| - Plasma | 8% | 5% | Proteins, coagulation factors |
| - Transcellular | ~1-2% | 1% | CSF, synovial, pleural, GI fluids |
What is Total Body Water?
Total Body Water (TBW) is the total amount of water contained in the body, distributed among various fluid compartments. Understanding TBW is crucial for fluid management, electrolyte calculations, and drug dosing in clinical medicine.
Body Composition
Water makes up 50-70% of body weight, varying by age, gender, and body composition. Muscle is ~75% water; fat is only ~10% water.
Fluid Distribution
TBW is divided into intracellular (2/3) and extracellular (1/3) compartments. ECF further divides into plasma and interstitial fluid.
Clinical Applications
TBW is essential for calculating sodium deficits, free water deficits, drug volumes of distribution, and dialysis adequacy.
How to Estimate Total Body Water
Formula Selection Guide
- 1
Watson Formula (Adults)
Most accurate for adults, accounts for age. Use for sodium calculations and general TBW estimation.
- 2
Hume-Weyers Formula
Alternative for adults, may be more accurate in certain populations. Does not account for age.
- 3
Simplified Formula
Quick bedside estimate: 0.6 x weight (males) or 0.5 x weight (females). Adjust for obesity/elderly.
- 4
Mellits-Cheek (Pediatric)
Validated for children. More accurate than adult formulas in pediatric population.
- 5
Adjust for Body Type
Obese patients: use lower TBW factor or lean body mass. Elderly: use reduced factor (0.5/0.45).
When to Calculate Total Body Water
Sodium Disorders
Required for sodium deficit calculations in hyponatremia and free water deficit in hypernatremia.
Drug Dosing
Hydrophilic drugs (aminoglycosides, vancomycin) distribute in TBW - loading doses based on TBW.
Dialysis Planning
TBW used for calculating Kt/V and urea reduction ratio (URR) in dialysis adequacy.
Fluid Resuscitation
Understanding TBW helps predict fluid distribution during resuscitation.
Body Composition
Research and clinical assessment of hydration status and body composition.
Critical Care
Fluid balance calculations in ICU, burn resuscitation, and volume overload states.
TBW Formulas
1. Watson Formula (Male)
Height in cm, weight in kg. Most accurate for adult males.
2. Watson Formula (Female)
Note: Female formula does not include age term
3. Hume-Weyers Formula
Male: TBW = (0.194786 x height) + (0.296785 x weight) - 14.012934
Female: TBW = (0.34454 x height) + (0.183809 x weight) - 35.270121
Alternative formula, may be more accurate in certain populations
4. Simplified Formula
Quick bedside estimate. Adjust factor for elderly (0.5/0.45) and obese (0.5/0.4).
Frequently Asked Questions
Why do women have lower TBW percentage than men?
Women typically have higher body fat percentage than men, and fat tissue contains only ~10% water compared to muscle's ~75% water. This results in a lower overall TBW percentage in women (45-55%) compared to men (55-65%).
How does obesity affect TBW calculations?
In obese patients, using total body weight with standard formulas overestimates TBW because fat has low water content. Options include: (1) using a lower TBW factor (0.4-0.5), (2) using lean body mass instead of total weight, or (3) using adjusted body weight formulas.
Which formula should I use for sodium calculations?
The Watson formula is most commonly used and validated for sodium disorder calculations (Adrogue-Madias formula). For bedside estimates, the simplified formula (0.6/0.5 x weight) is acceptable but may be less accurate in extremes of age or body composition.
Why does TBW decrease with age?
Aging is associated with decreased muscle mass (sarcopenia) and increased fat mass. Since muscle contains much more water than fat, this shift in body composition leads to decreased TBW percentage. Elderly patients are thus more susceptible to dehydration and electrolyte disturbances.
How do I calculate drug loading doses using TBW?
For hydrophilic drugs that distribute in TBW (like aminoglycosides): Loading Dose = TBW x Target Concentration. For vancomycin, loading doses of 25-30 mg/kg use total body weight in non-obese patients, but TBW-based or adjusted body weight calculations in obese patients.
Clinical Pearls
ICF vs ECF Electrolytes
ICF is high in K+ (140 mEq/L), Mg2+, and phosphate. ECF is high in Na+ (140 mEq/L), Cl-, and HCO3-. Sodium distributes primarily in ECF, potassium primarily in ICF.
Third Spacing
Fluid can shift to non-functional compartments (ascites, edema, bowel wall) - this "third space" fluid is unavailable for circulation despite adequate TBW.
Starling Forces
Fluid moves between plasma and interstitium based on hydrostatic and oncotic pressure gradients. Low albumin = low oncotic pressure = interstitial edema.
IV Fluid Distribution
1L NS stays primarily in ECF. 1L D5W distributes across TBW (only 1/3 stays in ECF). For rapid volume expansion, use crystalloid; for free water, use D5W.
Related Calculators
Free Water Deficit
For hypernatremia treatment
Sodium Deficit
For hyponatremia treatment
Sodium Change Calculator
Predict IV fluid effects
Serum Osmolality
Calculate osmolality
IV Flow Rate
Infusion rate calculations
GFR Calculator
Kidney function assessment
Creatinine Clearance
CrCl estimation
BMI Calculator
Body mass index
For informational purposes only — not medical advice. Consult a healthcare professional before acting on results.
🏥 Health Facts
— WHO
— CDC
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