Most of the water in your body is stored inside your cells. About two-thirds of your total body water sits within cells themselves, while the remaining third fills the spaces between cells and circulates as part of your blood. For an average person whose body is roughly 60% water by weight, that works out to about 40% of total body weight locked inside cells and 20% outside them. This distribution, sometimes called the 60-40-20 rule, shifts constantly depending on what you eat, your hormone levels, and how active you are.
Inside Your Cells
The single largest reservoir of water in your body is the fluid inside your cells, known as intracellular fluid. Every cell in your body, from muscle fibers to neurons, is essentially a tiny water-filled compartment. This water serves as the medium where all of the cell’s chemical reactions take place. It dissolves nutrients, carries waste, and maintains the structural shape of the cell itself.
Potassium is the dominant mineral inside cells, and it plays a key role in holding water there. Water naturally moves toward areas of higher mineral concentration, so the potassium inside cells acts as an anchor, pulling water inward. When your potassium levels drop or sodium levels rise, this balance shifts and water can move out of cells into surrounding spaces.
Between Cells and in Connective Tissue
The space between your cells is filled with interstitial fluid, which makes up the bulk of your extracellular water. This fluid bathes every tissue in your body, delivering nutrients from the bloodstream and carrying waste back out. It’s not just sitting in empty space. Much of it is bound up in a gel-like substance called ground substance, which is woven throughout your connective tissue, fascia, and the padding around your organs.
A molecule called hyaluronic acid is central to this process. It has an unusually strong ability to attract and hold water, creating a viscous, jelly-like environment in your connective tissues. When hyaluronic acid is fully active, its capacity to influence water distribution is disproportionately large relative to its size. This is the water that cushions your joints, keeps your skin plump, and allows layers of tissue to glide smoothly over each other. When you notice puffiness in your face, fingers, or ankles, it’s typically this interstitial fluid that has increased.
In Your Blood
Your blood is a significant water reservoir. Plasma, the liquid portion of blood, is about 91% to 92% water, and plasma itself makes up roughly 55% of total blood volume. This water carries proteins, hormones, glucose, and electrolytes throughout your body. It also plays a direct role in blood pressure: more water in your plasma means higher blood volume, which means higher pressure against your vessel walls.
Sodium in your blood acts like a sponge for water. Because sodium is concentrated in the extracellular space (including plasma), it draws water out of cells and into the bloodstream through osmosis. This is why a salty meal can temporarily increase your blood volume and leave you feeling bloated. Proteins in your plasma also exert a small pulling force that keeps water from leaking out of blood vessels into surrounding tissues.
Bound to Glycogen in Muscles and Liver
One of the most noticeable sources of water weight fluctuation is the water bound to glycogen, your body’s stored form of carbohydrate. Glycogen is packed into your muscles and liver, and it doesn’t travel light. Each gram of glycogen stored in muscle binds roughly 3 to 4 grams of water. In the liver, estimates range from about 1.6 to 3.8 grams of water per gram of glycogen.
This relationship explains why low-carb diets produce such dramatic early weight loss. When you cut carbohydrates, your body burns through its glycogen stores within a day or two, and all the water bound to that glycogen gets released. A person can store around 400 to 500 grams of glycogen between their muscles and liver, which means the water attached to it can easily account for 3 to 4 pounds. Reload on carbs, and that water comes right back. It’s not fat loss or fat gain. It’s a predictable fluid shift tied directly to how much glycogen your body is holding.
How Sodium and Hormones Control the Balance
Your kidneys are the main control center for how much water stays in your body versus how much gets excreted. Two hormones do most of the work. Aldosterone signals your kidneys to reabsorb sodium, and water follows the sodium back into your bloodstream. Antidiuretic hormone (sometimes called vasopressin) makes your kidney’s collection tubes more permeable to water, allowing even more to be reclaimed. These hormones ramp up when your blood pressure drops, when you’re dehydrated, or when your body senses low sodium levels.
Stress also plays a role. Your sympathetic nervous system (your “fight or flight” response) can trigger the same hormonal cascade, prompting your kidneys to hold onto sodium and water even when you don’t necessarily need it. This is one reason chronic stress can contribute to persistent puffiness or bloating.
For people who menstruate, the hormonal shifts across the cycle create a predictable pattern of fluid retention. Fluid retention peaks around the first day of menstrual bleeding and is significantly higher during the days surrounding your period compared to mid-cycle. The effect varies from person to person, but it’s a real, measurable shift driven by changes in estrogen and progesterone that influence how your kidneys handle sodium.
Why Inflammation Causes Local Swelling
When tissue is injured or inflamed, the tiny blood vessels in that area become more permeable. Their walls loosen, allowing protein-rich fluid to leak from the bloodstream into the surrounding tissue. This is the swelling you see around a sprained ankle or a bug bite. The extra fluid dilutes harmful substances and delivers immune cells to the area, but it also adds temporary water weight that’s concentrated in one spot.
Widespread inflammation, whether from illness, surgery, or a systemic immune response, can cause this leaking to happen across large areas of the body simultaneously. The result is generalized puffiness, particularly in the hands, feet, and face where gravity and loose tissue structure allow fluid to pool.
How Much Water Weight Can Fluctuate
Daily weight fluctuations of about 5 to 6 pounds are normal for healthy adults, roughly 2 to 3 pounds in either direction from your baseline. Water retention is the most common driver of these short-term swings. A high-sodium dinner, a heavy carb meal, a hard workout, hormonal shifts, or even a long flight can all temporarily increase the amount of water your body holds.
These fluctuations move water between the compartments described above. A salty meal pulls more water into your blood and interstitial spaces. A carb-heavy meal packs water into your muscles alongside glycogen. Hormonal changes redirect water from your bloodstream into surrounding tissues. None of these represent actual changes in body fat. They’re just water moving to different storage sites based on the signals your body is receiving at that moment.