Body Water Calculator — Total Body Water Percentage
Estimate how many litres of water your body contains and what percentage of your body weight is water — using the clinically validated Watson (1980) formula.
Sex
years
kg
cm
Watson (1980) formula estimate
- 1
Age term (0.09156 × age)
0.09156 × 30 = 2.7468 - 2
Height term (0.1074 × height)
0.1074 × 178 = 19.1172 - 3
Weight term (0.3362 × weight)
0.3362 × 75 = 25.215 - 4
Total body water
2.447 − 2.7468 + 19.1172 + 25.215 = 44Watson (1980) formula for males; age reduces TBW as lean mass declines.
How does this calculator work?
Total body water is estimated by the Watson formula: for men, TBW = 2.447 − 0.09156×age + 0.1074×height + 0.3362×weight; for women, TBW = −2.097 + 0.1069×height + 0.2466×weight (height in cm, weight in kg, result in litres). Healthy ranges are 50–65% for men and 45–60% for women of body weight.
Formula
How this is calculated
Total body water (TBW) is the sum of all fluid inside and outside your cells. It is estimated here with the Watson formula, a regression model published in 1980 that uses sex, age, height, and weight as predictors. The formula was derived from direct isotope-dilution measurements and remains widely used in clinical settings to guide fluid therapy and dialysis.
The result is expressed in litres and as a percentage of body weight. Healthy adults typically carry 50–65% water for men and 45–60% for women; men trend higher because muscle tissue holds more water than fat. The percentage falls with age as lean mass declines. The calculator also breaks the estimate into approximate compartments: about two-thirds of TBW sits inside cells (intracellular fluid, ICF) and one-third outside (extracellular fluid, ECF), of which roughly a quarter is blood plasma.
This is a population-average regression, not a direct measurement. Accuracy is typically ±5 litres versus gold-standard methods like deuterium dilution. It can be less accurate for extremes of adiposity, oedema, or age. For clinical decisions, always consult a qualified clinician.
Frequently asked questions
For healthy adults, roughly 50–65% for men and 45–60% for women. The figure is higher in lean, muscular individuals (muscle is about 76% water by weight) and lower with higher body fat (fat tissue is only about 10% water).
The Watson regression found that age significantly predicted TBW in men — lean body mass declines noticeably with age in men — but the effect was not statistically significant in the female cohort used to fit the model. Both sexes do lose body water with age in practice.
Building lean muscle mass raises the overall water fraction because muscle holds far more water than fat. Maintaining adequate daily fluid intake prevents chronic mild dehydration, which can decrease TBW by a percent or two.
Also known as
TG we-Calculate Editorial Team. (2026). Body Water Calculator — Total Body Water Percentage [Online calculator]. TG we-Calculate. https://we-calculate.com/calculator/body-water-calculator
TG we-Calculate Editorial Team. "Body Water Calculator — Total Body Water Percentage." TG we-Calculate. 2026. https://we-calculate.com/calculator/body-water-calculator.
TG we-Calculate Editorial Team, "Body Water Calculator — Total Body Water Percentage," TG we-Calculate, 2026. [Online]. Available: https://we-calculate.com/calculator/body-water-calculator
@misc{wecalculate_body_water_calculator, title = {Body Water Calculator — Total Body Water Percentage}, author = {{TG we-Calculate Editorial Team}}, howpublished = {\url{https://we-calculate.com/calculator/body-water-calculator}}, year = {2026}, note = {TG we-Calculate} }
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