Total Body Water Percentage Calculator

About this calculator

Total body water — the combined fluid inside cells, between cells, and in blood plasma — typically makes up somewhere around 45-60% of body weight, varying with age, sex, and body composition since muscle holds far more water than fat tissue does. The Watson formula, developed in 1980, is one of the standard equations for estimating it from simple measurements: height, weight, age, and sex, using separate coefficients for men and women.

Clinicians use total body water estimates to guide fluid management in dialysis, assess hydration status, and calculate drug dosing for medications that distribute through body water rather than fat. Nutritionists and sports-science professionals use it more informally to track hydration and body-composition changes. This calculator takes your height, weight, age, and sex and returns your estimated total body water and what percentage of your weight it represents.