Human excrement, commonly called feces or stool, is the solid waste your body expels after digesting food. It consists of everything your digestive system couldn’t absorb: water, dead bacteria, undigested fiber, fats, proteins, and shed cells from the intestinal lining. A healthy adult typically produces about 100 to 120 grams of stool per day, roughly a quarter pound, though people who eat high-fiber diets can produce around 150 grams daily.
What Feces Are Made Of
Stool is mostly water. In a healthy person, about 75% of feces by weight is water, with the remaining 25% being solid material. Of that solid fraction, 84% to 93% is organic matter. The rest is inorganic material like calcium and iron phosphates.
The organic portion breaks down further, and the numbers may surprise you. Somewhere between 25% and 54% of the solid matter is bacterial biomass, the remains of trillions of gut microbes that lived and died during digestion. Another 2% to 25% is protein and nitrogenous compounds (and roughly half of the bacterial mass itself is also protein). Undigested fats account for 2% to 15%. The remainder includes undigested plant fiber, shed intestinal cells, and bile pigments, which give stool its characteristic brown color.
How Your Body Forms Stool
Stool formation happens primarily in the large intestine, or colon. By the time partially digested food reaches the colon, most nutrients have already been absorbed in the small intestine. What arrives is a liquid mixture of water, fiber, bacteria, and waste products.
The ascending colon (the first section) absorbs the remaining water and electrolytes, gradually solidifying this liquid into stool. Water moves across the intestinal wall through osmosis, driven by the active absorption of sodium. Slow, rhythmic contractions called haustral contractions push the material forward while mixing it, giving the colon more time to extract water. The longer material stays in the colon, the more water gets absorbed and the firmer the stool becomes. This is why dehydration or slow transit time can lead to hard, dry stools, while rapid transit produces loose or watery ones.
The Bacteria Inside
Human feces contain an enormous concentration of bacteria, roughly 10 billion per gram of wet stool. These aren’t invaders. Most are beneficial microbes that help break down fiber, produce vitamins, and protect against harmful organisms. The composition varies significantly from person to person, but common species include Bifidobacterium, Eubacterium, and Ruminococcus, among many others.
This bacterial density is why stool is used in medical testing. The microbial profile of your feces reflects the health of your gut ecosystem, and changes in that profile can signal infections, inflammatory conditions, or dietary shifts.
What Healthy Stool Looks Like
The Bristol Stool Scale is a widely used medical tool that classifies stool into seven types based on shape and consistency:
- Type 1: Separate hard lumps, like pebbles
- Type 2: Lumpy and sausage-shaped
- Type 3: Sausage-shaped with cracks on the surface
- Type 4: Smooth, soft, and snakelike
- Type 5: Soft blobs with clear edges
- Type 6: Fluffy, mushy pieces with ragged edges
- Type 7: Entirely liquid, no solid pieces
Types 3 and 4 are generally considered ideal. Types 1 and 2 suggest constipation, meaning stool has spent too long in the colon and lost too much water. Types 6 and 7 indicate diarrhea, where stool moves through too quickly for adequate water absorption.
What Color Can Tell You
Normal stool ranges from light to dark brown, a color produced by bilirubin, a pigment created when your liver breaks down old red blood cells. Significant color changes can signal health issues:
- Green: Food moving through the intestines too quickly for bile to fully break down, or a bacterial infection
- Red: Possible bleeding in the lower digestive tract from hemorrhoids, ulcers, or fissures
- Black: Potential bleeding in the upper digestive tract (stomach or esophagus), though iron supplements and bismuth-based medications also cause black stool
- White, gray, or clay-colored: A possible problem with the liver, gallbladder, or pancreas, indicating that bile isn’t reaching the intestines
A single unusual bowel movement is rarely cause for alarm, especially after eating brightly colored foods like beets or leafy greens. Persistent color changes over several days are more meaningful.
Why It Smells
The distinctive odor of human excrement comes from volatile organic compounds produced by gut bacteria during fermentation. These include trace gases like indoles, phenols, organic acids, short-chain fatty acid alcohols, and sulfur-containing compounds such as hydrogen sulfide. The bulk gases in stool and flatulence (hydrogen, carbon dioxide, and methane) are actually odorless. It’s the tiny fraction of trace compounds that creates the smell. Diet plays a major role: high-protein diets tend to produce more indoles and sulfur compounds, intensifying the odor.
Pathogens in Human Waste
Beyond harmless gut bacteria, human excrement can carry dangerous infectious agents. This is the primary reason proper sanitation exists. Viruses commonly found in human feces include enteroviruses (including poliovirus), hepatitis A, rotaviruses, adenoviruses, astroviruses, caliciviruses, and coronaviruses. Bacterial pathogens like Campylobacter, Salmonella, and E. coli are also shed in stool during infections, along with parasites such as Giardia and Cryptosporidium.
These organisms spread through fecal-oral transmission, meaning contaminated water, unwashed hands, or improperly treated sewage can introduce them into food and drinking water. This is why waterborne diseases remain a leading cause of illness in regions without modern sanitation infrastructure.
How Sewage Systems Handle It
In modern municipal systems, human excrement travels through sewer pipes to wastewater treatment plants. The most common treatment method is activated sludge, an aerobic process that exposes the waste to oxygen-consuming microbes that break down organic matter. After treatment, the liquid portion is disinfected and discharged, while the solid residue becomes what the industry calls biosolids.
U.S. treatment plants generate over 13.8 million tons of biosolids (dry weight) annually. These nutrient-rich solids are sometimes applied to agricultural land as fertilizer, sent to landfills, or incinerated. Some facilities capture methane gas produced during the breakdown process and use it as an energy source, turning human waste into fuel.