Your body is roughly 60% water, contains about 30 trillion human cells, and is built from just a handful of chemical elements. But those simple ingredients combine into an extraordinarily complex system of molecules, cells, tissues, and organs that keeps you alive. Here’s what you’re actually made of, from atoms all the way up to organ systems.
Water: The Largest Single Component
Water is the most abundant substance in your body, though the exact percentage changes throughout your life. Newborns are about 75% water. By age 3 to 10, both boys and girls settle around 62%. From there, the numbers diverge: males hold relatively steady at around 60% through most of adulthood before dropping to about 57% after age 60. Females see an earlier decline to roughly 55% during puberty (as body fat increases and fat tissue holds less water than muscle), staying there through midlife, then dropping to about 50% after 60.
This water isn’t just sitting idle. It’s the solvent for every chemical reaction in your body, the medium that carries nutrients through your blood, and the fluid that cushions your brain and joints. Lose even 2% of your body water and you’ll start feeling thirsty, fatigued, and foggy.
The Six Elements That Make Up 99% of You
Strip away the water and zoom in further, and you’ll find that just six chemical elements account for nearly all of your body mass: oxygen, carbon, hydrogen, nitrogen, calcium, and phosphorus. Oxygen alone makes up about 65% of your weight, mostly because it’s a major component of water and is built into proteins, fats, and carbohydrates. Carbon forms the backbone of every organic molecule in your body. Hydrogen pairs with oxygen in water and appears in virtually every biological molecule. Nitrogen is essential to proteins and DNA. Calcium hardens your bones and teeth. Phosphorus shows up in bones, cell membranes, and the energy currency your cells run on.
Beyond those six, about a dozen trace elements play roles far out of proportion to their tiny quantities. Zinc supports your immune system. Iodine helps regulate metabolism. Copper is involved in handling oxygen in your tissues. Cobalt is a core part of vitamin B12, which your body can’t make on its own. Manganese, molybdenum, and several others are critical during prenatal development. Together, all these trace minerals account for less than 1% of your total body weight, but without them, essential chemical processes would grind to a halt.
Molecules: What Your Dry Weight Is Made Of
Once you account for water, the remaining “dry weight” of your body is dominated by four classes of large molecules. Proteins, DNA/RNA, and complex carbohydrates together make up 80 to 90% of your cells’ dry mass. Fats (lipids) are the other major component.
Proteins do most of the heavy lifting. They form the structural fibers in your muscles, the enzymes that speed up chemical reactions, the antibodies that fight infection, and the signaling molecules that let your cells communicate. Your body contains tens of thousands of distinct proteins, each folded into a precise shape that determines its job.
Fats serve as long-term energy storage, insulation, and the building material for every cell membrane in your body. They also cushion organs and help absorb certain vitamins. Carbohydrates provide quick energy and, in smaller amounts, serve as structural components and cell-surface markers that help cells recognize each other. Nucleic acids (DNA and RNA) carry your genetic instructions and manage protein production.
30 Trillion Cells, Plus a Few Trillion Guests
An average adult body contains about 30 trillion human cells. Males tend toward the higher end, around 36 trillion, while females average about 28 trillion, largely because of differences in overall body size. These cells come in more than 200 specialized types: red blood cells that carry oxygen, neurons that transmit electrical signals, muscle cells that contract, immune cells that patrol for threats, and many more.
Blood-forming cells dominate the total count. About 90% of all your human cells belong to the blood cell lineage, mostly red blood cells. That’s a bit misleading in terms of mass, though, because red blood cells are tiny. Your larger muscle and fat cells contribute far more to your weight despite being fewer in number.
You also carry roughly 38 trillion bacteria, most of them in your gut. For decades, textbooks claimed bacteria outnumbered human cells 10 to 1. Revised estimates put the real ratio at about 1.3 to 1, essentially a coin flip. And all those bacteria together weigh only about 0.2 kilograms, roughly half a pound. Still, this microbial community plays an outsized role in digestion, immune function, and even mood regulation.
Bones: A Surprisingly Dynamic Material
Your skeleton might feel like the most inert part of your body, but bone is a living, constantly remodeling tissue. By weight, bone is about 60% mineral (primarily a calcium-phosphorus crystal called hydroxyapatite), 30% protein, and 10% water. By volume, the proportions shift to roughly 40% mineral, 35% protein, and 25% water, because the mineral component is denser and packs more mass into less space.
About 90% of bone’s protein content is collagen, the same tough, flexible fiber found in skin and tendons. This combination of rigid mineral and flexible collagen is what gives bones their unique properties: hard enough to support your weight, yet flexible enough to absorb impact without shattering. Your skeleton also acts as a mineral bank. When your blood calcium drops, your body pulls calcium from bone to keep critical processes like nerve signaling and muscle contraction running.
How Your Organs Stack Up by Weight
Skeletal muscle is the single heaviest tissue type, making up roughly 36 to 42% of total body weight in an average person. Skin comes in as the largest organ by surface area. Fat tissue varies widely but typically accounts for 15 to 30% of body weight depending on sex and fitness level.
Your internal organs, despite their critical importance, are surprisingly light relative to the rest of you. The liver is the heaviest internal organ at about 1,300 grams in men (2.2% of body weight) and a bit more proportionally in women (2.4%). The brain weighs roughly 1,335 grams in men and 1,228 grams in women, representing about 2.3% of body weight, yet it consumes around 20% of your energy at rest. The heart, the organ you’d think would be substantial, weighs only about 270 to 280 grams, less than half a percent of your total weight. Each kidney weighs around 110 grams, and both lungs together come in under a kilogram. The spleen, often forgotten entirely, accounts for just 0.3% of body weight.
Putting It All Together
Your body operates on a kind of nested architecture. Atoms form molecules. Molecules build the internal structures of cells. Cells organize into tissues (muscle tissue, nerve tissue, connective tissue, epithelial tissue). Tissues combine into organs. Organs work together in systems: the cardiovascular system circulates blood, the digestive system extracts nutrients, the nervous system processes information, the immune system fights pathogens, and so on.
What makes this remarkable is how ordinary the raw materials are. The elements in your body are the same ones found in seawater, rocks, and the atmosphere. Carbon, oxygen, hydrogen, nitrogen, a handful of minerals, and a lot of water. The difference is organization. Those common ingredients, assembled into proteins, wrapped in membranes, coordinated across trillions of cells, and running on electrochemical signals, produce something that can think about what it’s made of.