Body Water Calculator
Find your total body water.
Result
How it is calculated
The tool estimates total body water as a fixed share of body weight: water (L) = weight (kg) × percentage / 100, with 60% used for males and 50% for females. Because 1 litre of water weighs about 1 kg, kilograms of water and litres are treated as equal. Weight must be greater than zero. The percentage is a population average, not a measurement: real values vary with age, body fat and muscle, since fat tissue holds much less water than lean tissue. Use the result as a rough estimate, not as a clinical reading. The litres are shown to two decimals, together with the percentage applied.
Worked example
A female with a body weight of 70 kg is entered. The tool uses 50% for females. Water = 70 × 50 / 100 = 35.00 L. The percentage shown beside it is 50.00%. For a male of the same weight the share would be 60%, giving 42 L, which shows how much the sex setting alone changes the estimate.
Common mistakes
- Entering weight in pounds; the formula expects kilograms.
- Reading the result as the amount of water you should drink per day. It is the water already in the body.
- Expecting a precise value for athletes or people with high body fat, since only sex and weight are used.
- Leaving the sex setting on the default and getting a figure 10 percentage points off.
About this calculator
This calculator estimates total body water (TBW) — the amount of fluid inside cells, blood and tissue — using the Watson formula, which factors in weight, height, age and sex (men typically carry a higher percentage of body water than women, around 60% versus 50%, due to differing muscle-to-fat ratios). Muscle tissue holds far more water than fat tissue, which is why the formula needs body composition inputs rather than weight alone.
Athletes and coaches monitoring hydration status around training, dietitians assessing a client's fluid balance, and anyone tracking body composition changes alongside weight loss use this figure as a baseline, since body water percentage shifts noticeably as fat and muscle mass change even when total weight stays the same.
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