Total Body Water Percentage Calculator

About this calculator

Total body water (TBW) is the fraction of your body weight that is actually water, and it matters clinically for hydration assessment, drug dosing, and dialysis planning. The Watson formula, published in 1980, estimates TBW in litres from sex, age, height and weight alone, offering a quick alternative to bioelectrical impedance analysis when that equipment isn’t available.

Physicians use it in nephrology and dialysis dosing, while sports scientists and dietitians use it to judge hydration status in athletes and clients. Normal TBW percentage differs meaningfully by sex — typically around 60% in men versus 50% in women — because muscle tissue holds far more water than fat tissue does.

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