Body Water Watson
Calculator

It estimates total body water, the sum of fluid inside and outside your cells, from height, weight, age, and sex using regression equations derived from measured body composition data. It gives a value in liters representing roughly half to two-thirds of body weight.

Men typically carry a higher proportion of lean mass and lower body fat percentage than women at the same weight, and since water is stored mostly in lean tissue, this changes the coefficients needed for an accurate estimate. Using the wrong sex-specific formula will systematically over- or underestimate body water.

It's used clinically to estimate fluid distribution for medication dosing and hydration assessment, and by athletes to contextualize sweat rate and hydration testing relative to total body fluid volume. Tracking changes over time can also help interpret shifts in weight that reflect water rather than fat.

Each formula was derived from a different reference population and set of regression variables, so they don't produce identical numbers for the same inputs. Differences are usually small, a liter or two, but you should stick to one formula when tracking changes over time for consistency.

Yes, indirectly, since the formula's coefficients were built assuming typical body composition for a given height, weight, age, and sex. Someone with unusually high or low body fat for their stats will have an actual body water percentage that deviates from the Watson estimate.