Total Body Water Percentage Calculator

About this calculator

Total body water (TBW) — the litres of fluid inside and outside your cells — is most commonly estimated with the Watson formula, which uses age, height, weight and sex because body composition (and therefore water content) differs systematically between men and women and shifts with age: for men, TBW = 2.447 − (0.09156 × age) + (0.1074 × height in cm) + (0.3362 × weight in kg); for women, TBW = −2.097 + (0.1069 × height in cm) + (0.2466 × weight in kg). Dividing TBW by body weight gives the percentage figure, which typically runs around 50–65% for adults, higher in lean, muscular bodies since muscle holds far more water than fat.

Clinicians use TBW estimates to calculate a free water deficit in dehydrated or hypernatremic patients, to dose hydrophilic medications that distribute through body water rather than fat, and to guide fluid management in dialysis and critical care, where knowing a patient’s water compartment size directly changes the treatment plan. Athletes and dietitians also use it in a lighter-weight way, as a rough marker of hydration status and lean body mass alongside other measurements.

This calculator takes your age, height, weight and sex and returns your estimated total body water in litres and as a percentage of body weight using the Watson formula.