Water makes up about 60% of an adult man’s body weight and 52 to 55% of an adult woman’s. That single number shifts quite a bit depending on your age, body composition, and how much muscle versus fat you carry. Here’s what shapes that percentage and why it matters.
How Water Percentage Changes With Age
Newborns are roughly 75% water by weight, making them far more water-dense than adults. Over the first year of life, that percentage drops steadily as body fat increases and organ proportions shift. By around age 12, children reach adult water levels.
On the other end of the spectrum, adults over 65 see their total body water decline further. This happens because aging reduces both total body mass and the proportion of lean tissue, which holds far more water than fat. It’s one reason older adults are more vulnerable to dehydration: they simply have less of a water reserve to draw on.
Why Men and Women Differ
The gap between men (around 60%) and women (52 to 55%) comes down to body composition. Fat tissue contains only about 14% water, while fat-free tissue (muscle, organs, bone) is roughly 80% water. That’s nearly a sixfold difference. Because women on average carry a higher proportion of body fat, their overall water percentage is lower. For the same reason, a very lean person of either sex will have a higher body water percentage than someone with more stored fat, regardless of total weight.
Where the Water Actually Sits
Your body’s water isn’t just sloshing around freely. About 62% of it sits inside your cells, forming the medium where virtually all cellular chemistry takes place. The remaining 38% is extracellular: it’s in your blood plasma, in the fluid between cells, and in specialized fluids like cerebrospinal fluid and the liquid inside your eyes.
This balance matters. Your body tightly regulates the ratio between intracellular and extracellular water using dissolved salts (mainly sodium and potassium). When you’re dehydrated or overhydrated, these compartments shift, and that’s what triggers symptoms like dizziness, confusion, or swelling.
What Water Does in Your Body
Water isn’t just filler. It performs specific, essential jobs:
- Temperature regulation. Sweating and the evaporation of moisture from your lungs are your primary cooling systems. Both require water.
- Nutrient and oxygen transport. Your bloodstream, which is mostly water, carries dissolved nutrients, minerals, and oxygen to every cell.
- Waste removal. Your kidneys and liver rely on adequate water to filter and flush metabolic waste through urine.
- Joint lubrication. The fluid in and around your joints reduces friction and absorbs impact.
- Shock absorption. Water cushions your brain, spinal cord, and (during pregnancy) the developing fetus.
- Digestion. Saliva, stomach acid, and intestinal fluids all depend on water to break down and move food.
How Much Water You Need Daily
Total fluid needs for a healthy adult fall in the range of 11.5 cups (2.7 liters) for women to 15.5 cups (3.7 liters) for men. That includes all fluid sources: drinking water, other beverages, and the water naturally present in food (fruits, vegetables, soups, and even bread contribute meaningful amounts). Most people who eat a varied diet and drink when thirsty come reasonably close to these numbers without tracking.
Your needs go up with exercise, heat exposure, fever, or illness that causes vomiting or diarrhea. Pregnancy and breastfeeding also increase fluid demands.
When Water Loss Becomes a Problem
Dehydration is clinically defined by the percentage of body weight you’ve lost as water. In infants, losing as little as 5% of body weight in fluid is considered mild dehydration. In older children, the threshold is around 3%. For adults, losing just 1 to 2% of body weight in water is enough to cause noticeable symptoms: thirst, darker urine, fatigue, and reduced concentration.
Your body compensates for small losses by concentrating urine and triggering thirst, but these signals become less reliable with age. Older adults often don’t feel thirsty until they’re already mildly dehydrated, which is why consistent fluid intake throughout the day matters more as you get older. The color of your urine is a more reliable gauge than thirst alone: pale yellow generally signals adequate hydration, while dark amber suggests you need more fluid.