Estimate total body water with the Watson formula. View body-water percentage plus approximate intracellular and extracellular fluid splits for reference.
The Body Water Percentage Calculator estimates total body water (TBW) with the Watson formula, a commonly used anthropometric equation based on sex, age, height, and weight. Water is the largest single component of the body, but the exact percentage varies substantially with body composition, age, and recent hydration status.
This page converts the Watson estimate into liters, percent of body weight, and an approximate intracellular fluid (ICF) and extracellular fluid (ECF) split. Those ICF and ECF values are rough reference proportions rather than direct measurements.
The result is best used as a body-composition or hydration-context estimate. It is not a direct hydration test and does not replace tracer-based measurement, bioimpedance, or clinical assessment when precise fluid status matters.
This calculator is useful when you want a rough total-body-water estimate from standard body measurements. The result can help with body-composition context, hydration discussion, and understanding why leaner and younger people often have a higher body-water percentage than older or higher-fat-mass individuals.
Watson Formula — Males: TBW = 2.447 - 0.09156 x age + 0.1074 x height(cm) + 0.3362 x weight(kg). Females: TBW = -2.097 + 0.1069 x height(cm) + 0.2466 x weight(kg). Intracellular Fluid is shown as about 60% of TBW. Extracellular Fluid is shown as about 40% of TBW. Body Water % = (TBW / weight) x 100.
Result: TBW ≈ 45.3 L (56.6% of body weight)
Using the Watson formula for males: TBW = 2.447 - 0.09156 x 35 + 0.1074 x 178 + 0.3362 x 80 = about 45.26 L. That is about 56.6% of body weight (45.26/80). The page then shows ICF at about 27.2 L and ECF at about 18.1 L using a simple proportional split.
Water supports circulation, temperature regulation, nutrient transport, and waste removal. Because it makes up such a large share of body mass, even modest shifts in fluid balance can change body weight and symptoms noticeably.
Total body water estimates appear in a variety of medical and research calculations, including some toxicology, nephrology, and electrolyte-context formulas. In those settings, the estimate is one input among many rather than a stand-alone treatment instruction.
Sex, age, lean mass, fat mass, training status, illness, and recent hydration all influence body water content. That is why a formula-based estimate should be read as a broad approximation rather than as a precise reflection of moment-to-moment fluid status.
Tracer-dilution methods such as deuterium dilution are classic reference methods for measuring total body water directly. Bioimpedance can also provide practical estimates in some settings. The Watson formula is less direct than those approaches, but it remains useful when only routine anthropometric data are available.
Last updated:
This calculator uses the Watson anthropometric equations to estimate total body water from age, sex, height, and weight. It then converts the estimate into liters, body-water percentage, and a rough intracellular/extracellular split for display.
The Watson equation is an estimate, not a direct fluid measurement. The ICF and ECF values shown on the page are visual reference proportions only, so they should be treated as broad context rather than measured compartment volumes.
For adult males, body water percentage is often cited around 50% to 65% of body weight. For adult females, it is often cited around 45% to 60%. These are broad reference ranges rather than strict clinical cutoffs.
Aging is often accompanied by lower lean mass and relatively higher fat mass. Because lean tissue contains much more water than fat tissue, total body water as a percent of body weight often declines over time.
Body-fat level is one of the biggest drivers of body-water percentage. Two people of the same height and weight can have noticeably different water percentages if their lean-mass and fat-mass distribution differs.
The Watson formula is a sex-specific anthropometric equation published in 1980 to estimate total body water from age, height, and weight. It was developed for pharmacokinetic work and remains a practical estimate when direct measurement is not available.
Intracellular fluid (ICF) is the water inside cells, while extracellular fluid (ECF) refers to water outside cells, including plasma and interstitial fluid. This page shows them as approximate proportions of total body water rather than directly measured compartments.
Daily intake targets vary with body size, food intake, environment, and activity. Population-level intake guides can be useful for context, but they are not individualized hydration prescriptions.
Yes, actual total body water changes with hydration state. The Watson formula does not directly measure acute dehydration, though, so it is better interpreted as an expected TBW estimate from body measurements than as a real-time hydration test.
Yes, it is used as an estimate in some pharmacokinetic and nephrology contexts. It is better viewed as a practical approximation than as a direct fluid-status measurement.