Your body is mostly water, but exactly how much varies from person to person. Water sits inside your cells, in your blood, and in the fluid between cells, and the total depends on your size, age and body composition. This estimator gives you a quick, plain number for that total, using only four inputs: your sex, age, height and weight. It is a teaching tool, not a health test, and understanding what the number can and cannot say is the whole point.
The calculation uses the Watson formula, published in 1980 and still widely used as a teaching estimate. For men, total body water in litres is 2.447 minus 0.09156 times age, plus 0.1074 times height in centimetres, plus 0.3362 times weight in kilograms. For women, it is minus 2.097 plus 0.1069 times height plus 0.2466 times weight. The tool divides the result by your weight and multiplies by one hundred to give your body water as a percentage of body weight.
Here is a worked example. A thirty-year-old man who is 175 centimetres tall and weighs 75 kilograms gets 2.447 minus 2.747 plus 18.795 plus 25.215, which is about 43.7 litres. Divided by 75 kilograms, that is about 58.3 percent body water. A woman of the same age, height and weight gets minus 2.097 plus 18.708 plus 18.495, which is about 35.1 litres, or about 46.8 percent. The difference is expected: on average, women carry a higher proportion of body fat, and fat tissue holds far less water than muscle.
Typical adult values fall roughly between 45 and 65 percent, with younger adults and leaner, more muscular people toward the higher end, and older adults toward the lower end. Age matters because muscle mass tends to fall with age while fat proportion rises, and both shifts lower the water percentage. That is why the formula includes age for men and why two people of the same weight can have very different results: a muscular 75 kilograms and a less muscular 75 kilograms are not the same body.
It is important to know what this estimate is not. It is not a measurement of your hydration today. Drinking a litre of water does not change your total body water percentage in any lasting sense, and the formula cannot tell whether you are dehydrated after exercise, illness, or a hot day. Real hydration assessment uses symptoms, blood tests, and clinical judgement, not a height and weight formula. If you feel unwell, dizzy, confused, or unable to keep fluids down, seek professional care rather than checking a calculator.
So why is the number useful at all? It gives students, trainers and curious readers a realistic sense of scale: an average adult carries roughly forty litres of water, more than most people guess, and that water is doing constant work carrying nutrients, regulating temperature, and cushioning joints. It also shows why one-size advice fails: a small older adult and a large young athlete have very different water totals, so a fixed daily water target copied from the internet cannot suit both. The formula makes that variation visible with simple maths.
Use this estimator as a learning aid only. Enter honest numbers, read the percentage alongside the range note, and remember that the Watson formula describes an average person of your measurements, not you specifically. It offers no diagnosis, no treatment, and no personal drinking recommendation, and it should never delay professional advice about a health concern. As an educational estimate, though, it turns an abstract fact, that you are mostly water, into a concrete number you can picture and understand. Keep the result in perspective: it is one average for your measurements, and your real value could sit several points either side without anything being wrong.