These are rough estimates used in routine clinical settings:
Men:
TBW ≈ 60% of body weight
Women:
TBW ≈ 50–55% of body weight
Elderly or obese individuals:
Lower percentages are used (e.g., 45–50%)
Example:
A 70 kg male → TBW ≈ 0.6 × 70 = 42 liters
Knowing Total Body Water (TBW) helps guide fluid replacement therapy in dehydration by:
Dehydration involves a loss of body water. By knowing a person's TBW, clinicians can quantify how much water has been lost and how much is needed to restore balance.
Example Calculation:
A 70 kg man (TBW ≈ 60%) has about 42 L of TBW.
If he's estimated to be 5% dehydrated, then fluid deficit ≈ 5% of TBW:
→ 0.05 × 42 L = 2.1 L fluid deficit
TBW helps tailor fluid therapy based on:
Degree of dehydration (mild: 3–5%, moderate: 6–9%, severe: ≥10%)
Patient characteristics (age, weight, sex, comorbidities)
Fluid type (oral rehydration vs. IV crystalloids like normal saline or lactated Ringer's)
Replacing too little → persistent hypovolemia, organ hypoperfusion
Replacing too much → fluid overload, especially dangerous in heart or kidney disease
TBW estimates guide:
How much to replace in initial resuscitation (e.g., 1–2 L bolus in moderate-severe dehydration)
How much to give over maintenance and correction phases
When to slow or stop rehydration
A child with 10% dehydration and a weight of 20 kg:
Estimated TBW = 60% × 20 = 12 L
Deficit = 10% × 12 = 1.2 L → Replace this gradually over 24–48 hours, depending on the severity and setting.