Work out total body water instantly with clear inputs, formula shown and shareable results.
The Watson equations predict total body water from weight, height, age and sex, and remain the clinical standard for dosing and dialysis calculations. A 40 year old man of 75 kg and 175 cm holds about 42.5 L, roughly 57% of body weight. Water is lost with age as muscle mass declines, which is why age carries a negative coefficient.
Watson, men
TBW = 2.447 - 0.09516 x age + 0.1074 x height(cm) + 0.3362 x weight(kg)
Watson, women
TBW = -2.097 + 0.1069 x height(cm) + 0.2466 x weight(kg)
These figures are population-average estimates, not a medical assessment or diagnosis. Body composition equations carry errors of several percentage points, and healthy ranges vary with age, ethnicity, muscularity and medical history. Discuss weight or body composition goals with a qualified clinician or dietitian.
Because in Watson's original regression age did not add predictive value for women once height and weight were included.
Ethanol and lithium dosing, dialysis adequacy (Kt/V) and fluid replacement planning all depend on it.