Here is a puzzle in the numbers. At any one moment the gut holds on average less than 200 milliliters of gas, yet people pass about 600 to 700 milliliters a day. By that arithmetic, a person passes at least three times as much gas in a day as the gut holds at any single moment. Even so, there is always gas in the stomach and intestines. Everyone has it at all times, and swallowed air stays there until it is passed. Gas in the digestive tract has two main sources: air that is swallowed, and gas made when bacteria in the large intestine break down food. So where does all that gas come from?
The two sources can be told apart by what they contain. Gas is a mix of gases: carbon dioxide, nitrogen, oxygen and hydrogen, and sometimes methane, and most of these have no smell. Swallowed air is mostly nitrogen and oxygen. Gas made by bacteria from food is mostly carbon dioxide, hydrogen and sometimes methane. Some research labs use that difference by counting how often people pass gas and testing what it contains. A mix rich in nitrogen points to swallowed air, while more carbon dioxide, hydrogen and methane point to bacteria working on food. That is the logic behind reading a sample of gas for clues about where it came from.
Start at the upper end, where the story is simplest. Belching releases gas from the upper digestive tract, the esophagus and stomach, while passing gas, called flatulence, happens in the lower digestive tract. Most belching comes from swallowed air, and that air most often never reaches the stomach and instead builds up in the esophagus, the tube that leads to the stomach. As of November 2026, people typically belch up to 30 times a day, and burps come most often soon after eating. Belching during or after meals is normal. Swallowing air has a medical name, aerophagia, and some people burp a lot because they swallow too much air and release it before it reaches the stomach. Belching, in other words, is the upper digestive tract letting go of air that never travelled far.
What makes a person swallow extra air? People swallow extra air when they eat or drink too fast, talk while eating, chew gum, suck on hard candies or drink carbonated drinks. Chewing gum and sucking on hard candy make you swallow more often than normal, and part of what you swallow is air. Carbonated drinks and beer are made fizzy with carbon dioxide gas. When you drink them, the gas is released inside you and can come back up as burps. Taking your time over a meal, and not gulping or talking while eating, means you swallow less air. Most swallowed air comes back up as burps, and very little of it goes on to the rest of the digestive system; a little can skip the burp and move on into the intestines, but most of that small amount is soaked up into the blood.
The lower end is the work of microbes. The large intestine holds a large number of bacteria, plus some fungi and viruses, that together make up the gut microbiome; two big groups of bacteria, Bacteroidetes and Firmicutes, are the main gas producers in the gut and make up over 90 percent of its bacteria. Among the most abundant microbes that make gas in the intestine are Bacteroides, Ruminococcus, Roseburia, Clostridium, Eubacterium, Desulfovibrio and Methanobrevibacter; people can differ in how much gas they make partly because they have more or different bacteria living in their digestive tract, which helps explain why gas amounts differ from one person to the next. Most of the gas that comes out of the bottom is made by these bacteria as they break down food, not swallowed.
What the bacteria work on is food the body could not finish, and the supply of that food decides much of what follows. Food carbohydrates come as sugars, starches and fiber, and the stomach and small intestine cannot fully digest some of them, so they travel on to the large intestine. There, bacteria ferment them: they break down food that was not digested earlier and give off gas as a byproduct. The longer food stays in the large intestine, the more time bacteria have to ferment it, so more gas can build up; fat slows digestion, which gives food more time to ferment. Gas is only one of the products of this fermenting, because the bacteria also release short-chain fatty acids and many other compounds. Tiny helpful bacteria in the large intestine do the job that the earlier parts of the gut could not.
Foods differ in how much of their carbohydrate goes unabsorbed: about 20 percent in baked beans, 7 to 10 percent in wheat, oats, potatoes and corn, and only 1 percent in white rice; the unabsorbed part is what bacteria get to ferment. Beans and other legumes have a high share of carbohydrates that people cannot digest; they are full of sugars called oligosaccharides, and in one test, flour from soybeans bred to have less of these sugars produced less gas than flour from ordinary soybeans. Raffinose is a complex sugar found in beans, cabbage, Brussels sprouts, broccoli, asparagus, other vegetables and whole grains.
Fiber is a good test of the idea. Soluble fiber dissolves in water and becomes soft and gel-like, and it is found in oat bran, beans, peas and most fruits. Insoluble fiber, such as the fiber in wheat bran, passes through the intestines almost unchanged and makes little gas. In one study, a diet with no fiber cut the gas a person passed in a day from about 700 milliliters to about 200 milliliters, a drop to less than a third of the starting amount. Certain carbohydrates called FODMAPs are poorly absorbed and rapidly fermented by bacteria in the intestines, which makes more gas.
The gas that a person passes is not all the gas the bacteria make, and how it leaves is part of the story. Gas made by gut bacteria leaves by one of three routes: other microbes use it up and turn it into non-gas products, it is absorbed into the blood and breathed out, or it comes out the bottom as a fart; this is part of why a person passes less gas than the bacteria make. A lot of gas is absorbed into the blood or burped up before it ever leaves through the bottom. Human cells do not make hydrogen when they handle sugars, but gut bacteria do; so scientists can have a person drink a test sugar and then measure the hydrogen in their breath, which shows that hydrogen made in the gut reaches the blood and the lungs. A rise in breath hydrogen shows that gut bacteria have been fermenting the sugar.
Methane adds a twist. About one in three people make methane gas in the large intestine, along with hydrogen and carbon dioxide; some gut microbes, called methanogens, use hydrogen to turn carbon dioxide into methane. Because several gas molecules become one, their work can shrink the total volume of gas. About three quarters of fart gas, 74 percent, is hydrogen, methane and carbon dioxide made by bacteria in the large intestine as they ferment food. Gas made in the large intestine can burn, because it has hydrogen and methane in it.
Smell comes from a tiny slice of the total. About 1 percent of a fart is made of the gases that cause the odor, and gases that smell make up less than 1 percent of the gas in the gut; they are sulfur-containing trace gases such as hydrogen sulfide, and the other gases, which make up the rest, have no odor at all. Bacteria called sulfate reducers make hydrogen sulfide in tiny amounts, and the sulfate they use comes mostly from sulfur-containing amino acids, which are the building blocks of protein. Hydrogen sulfide is the waste of the microbes that help digest what the body cannot. Smelly gas also has other causes: compounds made when meat is digested, and whether stool is in the rectum at the time; foods high in sulfur, like proteins and cruciferous vegetables such as cauliflower, cabbage and broccoli, are usually behind foul-smelling gas.
In a study of how the colon works, scientists collected every fart from 16 people over 4 hours and ran each one through a gas chromatograph, a machine that sorts a gas mix into its parts and measures each; it measured nitrogen, oxygen, hydrogen, carbon dioxide, methane and the sulfur gases that cause odor. The mix of gases varied a great deal from one passage to the next in the same person, and varied enormously from person to person; that is why no single recipe describes a typical fart. The same study found that methane-making microbes and hydrogen-sulfide-making bacteria, which usually do not live together in a sample, were both present in the colon at once. Studying the gas people pass is a way to learn which microbes are working inside the unopened colon.
Gas does not leave the gut in a smooth stream. Researchers fed mixtures of nitrogen, oxygen and carbon dioxide into volunteers' intestines, and the gas came out the other end in pulses, which suggests the gut's own muscle patterns control its release. Other experiments found that liquid or solid meals make gas move through the gut faster. When you burp, two sphincters (ring-shaped muscles) take turns: measurements show the lower esophageal sphincter, near the stomach, loosens first, and then the upper esophageal sphincter, near the throat, relaxes and lets the gas out. A fart is a muscle teamwork event: pressure tests show waves of squeezing in the colon, rising pressure in the rectum, and the anal sphincter relaxing early, which lets gas out and can be done on purpose or without trying.
Counts of how much gas people pass depend on the source. As of November 2026, studies suggest people pass gas an average of 8 to 14 times a day, and one hospital says 13 to 21, while another source says most people make about 1 to 4 pints of gas a day and pass gas about 14 times a day. The amount passed is about half a quart to a quart and a half, or 0.5 to 1.5 liters, and some people pass gas more or less often than that. Experts consider passing gas up to 25 times a day to be normal, and there is great variability in how much and how often people pass gas.
Farting happens when bacteria in the large intestine work on parts of your food that your own body cannot digest. It is a normal process, and the gas has to go somewhere. Passing gas is rarely a sign of a medical issue, and healthy people make a lot of gas every day. What counts as a normal amount of farting is different for every person, and some people pass gas more than others, and that is normal. Holding a fart in does not make it disappear. The gas just builds up inside, which can make your belly feel full or puffy until it finds its way out, one way or another.
Words to know
- aerophagia: swallowing air
- methanogens: methane-making microbes
- microbiome: gut microbe community
- chromatograph: gas-sorting machine
- FODMAPs: fast-fermenting carbohydrates
