How to Use the Body Water Weight Calculator
Enter your sex, age, height and weight, and the body water weight calculator estimates your total body water in liters and as a percentage of your body weight. It also splits that water into what's inside your cells and what's outside them.
- Sex: male or female. Men and women have different equations because of average differences in muscle and fat.
- Age: in years. In men, body water falls slightly with age as muscle mass declines.
- Height: in centimeters or feet and inches.
- Weight: in kilograms or pounds, ideally a morning weight when you're normally hydrated.
Total body water is shown in liters; because a liter of water weighs a kilogram, it's also your water weight in kg. Share of body weight puts it in context: around 60% is typical for adult men and 50–55% for women. The ICF and ECF figures show the intracellular fluid (inside cells) and extracellular fluid (blood plasma and the fluid between cells).
How the Body Water Weight Calculator Works
The calculator applies the Watson formula, a regression equation that predicts total body water (TBW) from simple body measurements. It was built against isotope-dilution measurements, the laboratory reference method, and is widely used in clinical medicine.
Watson total body water formula
Men: TBW (L) = 2.447 − (0.09516 × Age) + (0.1074 × Height cm) + (0.3362 × Weight kg)Women: TBW (L) = −2.097 + (0.1069 × Height cm) + (0.2466 × Weight kg)Body water (%) = TBW ÷ Weight kg × 100 · ICF = ⅔ TBW · ECF = ⅓ TBW- Age in years (used in the male equation only).
- Height in centimeters; feet and inches are converted at 1 inch = 2.54 cm.
- Weight in kilograms; pounds are converted at 1 kg = 2.205 lb.
Worked example: A 40-year-old man is 178 cm tall and weighs 80 kg. TBW = 2.447 − (0.09516 × 40) + (0.1074 × 178) + (0.3362 × 80) = 2.447 − 3.806 + 19.117 + 26.896 = 44.7 L. That's 44.7 ÷ 80 × 100 = 55.8% of his body weight, with about 29.8 L inside cells and 14.9 L outside them.
The two-thirds/one-third split is a textbook approximation. The formula assumes normal hydration and typical body composition, so it can over- or underestimate in athletes, people with obesity, and anyone with swelling or dehydration. Results are rounded to 0.1 L and 0.1%.
Body Water Weight Calculator FAQs
What percentage of body weight is water?
In healthy adults, water typically makes up about 60% of body weight in men and 50–55% in women. The share is higher in infants (around 75%) and lean, muscular people, and lower in older adults and people with more body fat.
How is total body water calculated?
This calculator uses the Watson formula, which predicts total body water in liters from sex, age, height and weight. It was developed by comparing those measurements with laboratory isotope-dilution tests in hundreds of adults.
How accurate is the Watson formula?
For typical adults it usually lands within a few liters of lab measurements, which is good enough for general use and is why clinicians use it in dialysis dosing. It is less accurate for people with obesity, very high muscle mass, fluid retention (edema), pregnancy or children.
Is body water weight the same as water retention?
No. Total body water is the normal, healthy water inside and around your cells. Water retention is a short-term excess, often from salt, hormones or carbohydrates, that typically adds 1–2 kg and isn't estimated by this calculator.
Why do women have a lower body water percentage?
Women on average carry more body fat and less muscle than men. Muscle is about 75% water while fat tissue is only around 10–20%, so a higher fat share lowers the overall percentage.
Can I measure my body water at home?
Many bioelectrical impedance (BIA) scales estimate body water, but their readings shift with hydration, meals and exercise. For a consistent trend, weigh in at the same time each morning, or use this formula as a stable reference point.