How this calculator works
The Watson regressions estimate adult total body water in litres. Height is converted to centimetres and weight to kilograms. The displayed mass assumes approximately 1 kg per litre of water. The inverse modes are algebraic exercises; they do not measure age, body composition or hydration. The result is an estimate for information only and is not medical advice.
Formula
Male TBW = 2.447 - 0.09156 age + 0.1074 height + 0.3362 weight. Female TBW = -2.097 + 0.1069 height + 0.2466 weight. Age is years, height cm, weight kg, TBW L.
Worked example
A 40-year-old man at 175 cm and 70 kg has estimated TBW 41.1136 L, about 58.73% of weight. The female equation at the same height and weight gives 33.8725 L.
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Frequently asked questions
Why is age absent from the female equation?
The published female regression did not retain an age term. It cannot be rearranged to estimate age.
Does this detect dehydration?
No. This is an anthropometric estimate, and illness, oedema, body composition and population differences affect accuracy.
Can children use it?
This implementation is limited to adults. A paediatric body-water model needs its own validated coefficients.