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What is total body water (TBW)?
Total body water (TBW) is the sum of all the fluid in your body, and in an adult it makes up roughly more than half of body weight. Water is essential for cells to function, for transporting nutrients, regulating body temperature and eliminating waste. TBW is divided into two compartments: about two thirds is inside the cells (ICF) and one third is outside the cells (ECF). Body water percentage varies with sex, age and body composition; it is higher in men than in women, and higher in muscular individuals than in those with more body fat.
How is TBW calculated?
This tool uses the Watson (1980) equation, widely used in clinical practice. Total body water is estimated in liters by sex, using age, height and weight:
Female: TBW = −2.097 + 0.1069 × height + 0.2466 × weight
height (cm), weight (kg), result in liters (L)
Example: a man aged 35, 178 cm, 80 kg:
2.447 − 0.09156×35 + 0.1074×178 + 0.3362×80 ≈ 45.3 L
This corresponds to a water percentage of 45.3 ÷ 80 = 57% — within the healthy range of 50–65% for men.
Method: Watson PE et al. (1980) total body water estimation equations. Reference ranges: adult men approximately 50–65%, women 45–60% (as a percentage of body weight). The intracellular/extracellular distribution is approximately 2/3 – 1/3.
What you should know about water balance
- It is an estimate, not a measurement: the Watson formula is based on population averages; for an exact value, bioelectrical impedance (BIA) or clinical methods are needed.
- Inverse relationship with body fat: fat tissue holds little water, so as body fat rises the water percentage falls. A “low” result does not necessarily mean dehydration.
- Don't confuse it with daily water needs: TBW estimates the water currently in your body, not how many liters you should drink. For that, use the water intake tool.
- Too much and too little water are both harmful: both dehydration and excess water (hyponatremia) carry risks; monitor your balance with thirst and urine color.
Frequently Asked Questions
As a general reference, water makes up about 50–65% of body weight in adult men and 45–60% in women. The percentage is slightly lower in women because of their higher average body fat. It also declines with age.
Women have a higher average body fat percentage than men. Because fat tissue holds far less water than muscle, the total water percentage is typically slightly lower in women.
The Watson equation is a validated estimation method widely used in clinical practice; however, because it is based on population averages, individual variation is possible. For an exact measurement, bioelectrical impedance (BIA) or isotope methods are used.
TBW percentage depends largely on body composition: building muscle mass and lowering body fat raise the water percentage. Simply drinking more water does not meaningfully change the long-term percentage; the body eliminates the excess.
About two thirds of total water is inside the cells (ICF), and one third is outside the cells (ECF — blood plasma, tissue fluid). This balance is critical for health; when it is disrupted it appears as edema or dehydration.
No. TBW estimates the total water currently present in your body. Daily water need is how much you should drink and depends on weight, activity and climate — for that, use the separate water intake tool.