Body Water Percentage Calculator

Total body water (TBW) represents the total amount of water in the human body, expressed as a percentage of total body weight. Water is essential for virtually every bodily function including temperature regulation, nutrient transport, waste elimination, and cellular processes.

The Watson Formula:

This calculator uses the Watson formula, one of the most widely validated equations for estimating total body water:

For males: TBW (liters) = 2.447 - 0.09156 × age + 0.1074 × height(cm) + 0.3362 × weight(kg)

For females: TBW (liters) = -2.097 + 0.1069 × height(cm) + 0.2466 × weight(kg)

The percentage is then calculated as: Body Water % = (TBW in liters / weight in kg) × 100

Normal body water percentages:

• Adult males: 50-65% of body weight
• Adult females: 45-60% of body weight
• Infants: About 75% of body weight
• Elderly adults: 45-55% of body weight

Women generally have a lower body water percentage than men because they typically have a higher proportion of body fat, and fat tissue contains less water than muscle tissue.

What this number is, and what it is not

Worth being clear, because the two get confused constantly. The Watson equation takes your age, height, weight and sex and returns the volume of water your body typically holds at that size. It is a body-composition estimate, in the same family as a lean-body-mass formula.

It is not a hydration reading. There is no input on this page that could tell it whether you are dehydrated, because there isn’t one: drink a litre of water and the equation’s answer moves by about a third of a litre, purely because you got a kilogram heavier. A bathroom scale that claims to measure body water is doing something different again, passing a small current through you and inferring water from the resistance, and those readings swing with skin moisture, recent meals and time of day.

In people, a higher percentage means more lean mass, since muscle is roughly 75% water and fat only about 10%. But Watson cannot see your muscle. It assumes a typical build for your height, weight and age, so a muscular person and a soft one of the same size get the same answer, and the percentage it prints mostly tracks how heavy you are for your height. A light, tall body reads high; a heavy, short one reads low. If you want to know whether you are drinking enough, urine colour and thirst are more informative than any formula.

Why hydration matters anyway:

Even mild dehydration of 1-2% of body weight impairs cognitive function, physical performance and mood. Losing more than 5% is medically dangerous. None of that is measured here, but it is the reason people look the topic up.

Factors affecting body water:

• Body composition: Muscle tissue is about 75% water, while fat tissue is only about 10% water
• Age: Body water percentage decreases with age as muscle mass typically declines
• Physical activity: Athletes with more muscle mass tend to have higher body water percentages
• Climate and altitude: Hot climates and high altitude increase water loss through sweat and respiration

Daily water intake recommendation:

A common clinical rule of thumb is 30-35 ml of water per kilogram of body weight per day. The official figures are set per person rather than per kilo, and they count the water in food as well as drinks. The European Food Safety Authority (EFSA, 2010) sets 2.0 litres a day for women and 2.5 for men. The US National Academies (2004) set 2.7 and 3.7 litres, and expect food to supply about a fifth of it. Active people and those in hot climates need more.