A gas has no fixed shape or size, and in a closed container it spreads out to fill all the space. The gas in the digestive tract is air mixed with other gases, and it leaves through the mouth when a person burps or through the bottom when a person passes gas. Scientists still say the inner workings of the intestines, where the body and its microbes meet, are hard to understand, and measuring gases and how they change with diet can reveal what the gut microbes make and do. Each method for measuring gut gas carries a limit: it either measures the gases indirectly, as a breath test does, or it is highly invasive. You swallow a little air every time you eat and drink, and burping and passing gas are natural and common.
Counting from the outside gives figures that differ by source. As of November 2026, people typically belch up to 30 times a day, and experts consider passing gas up to 25 times a day to be normal. Studies suggest an average of 8 to 14 times a day for passing gas, and one hospital says 13 to 21. The volume is given as about half a quart to a quart and a half, which is 0.5 to 1.5 liters, and on average people pass about 600 to 700 milliliters of gas a day; there is great variability in how much and how often people pass gas, so a normal day for one person is different from a normal day for another. Timing matters too: burps are more likely soon after a meal, and many people feel a tight, full feeling in the chest or stomach just before a burp, a feeling that goes away once the gas is out; belching during or after meals is normal. Experiments found that liquid or solid meals make gas move through the gut faster, which matches what people notice.
The amount the gut holds and the amount it passes do not match. 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, which is at least three times as much. 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. Gut bacteria that ferment food can make a lot of hydrogen: one review reports that up to 1 liter of hydrogen gas can be produced in the gut in 24 hours. By that figure, hydrogen alone could exceed the whole 600 to 700 milliliters that people pass in an average day; most of the gas that comes out of the bottom is made by bacteria breaking down food, not swallowed.
Some gas made in the gut is used up before it can leave. Some gut microbes use up hydrogen, and these hydrogen-using bacteria stop it from building up in the colon, where it would slow the fermenting done by other microbes. Gut bacteria make carbon dioxide when they ferment food, and most of it is soaked up through the wall of the colon into the blood and then breathed out, so some gut gas leaves through your lungs instead of your bottom. Researchers suggest that eating gas-making foods regularly may increase the gas-consuming pathways of the gut microbes, so more of the gas is absorbed into the blood and breathed out; the breath test follows gas that leaves by the lungs. Human cells do not make hydrogen when they handle sugars, but gut bacteria do; a person drinks a test sugar, and scientists measure the hydrogen in the person's breath; a rise in breath hydrogen shows that gut bacteria have been fermenting the sugar, and it shows that hydrogen made in the gut has reached the breath.
One study of how the colon works analyzed the gas that was passed. 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. Swallowed air is mostly nitrogen and oxygen, and some research labs count how often people pass gas and test 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; 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.
Some measurements are of muscles rather than molecules. When a person burps, two sphincters, which are 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; gas does not leave the gut in a smooth stream: in experiments where gas was fed into the gut, gas collected from the rectum came out in pulses, which suggests the gut's own muscle patterns control the flow. Belching releases gas from the upper digestive tract, the esophagus and stomach, while passing gas, called flatulence, happens in the lower digestive tract.
Smell is a chemistry problem of its own. Gases that smell make up less than 1 percent of the gas in the gut, and the other gases, which make up the rest, have no odor at all. About 1 percent of a fart is made of the gases that cause the odor, and the common gases in it, like nitrogen and carbon dioxide, have no smell. The rotten-egg gas starts with food and microbes: sulfate can come from several foods, and gut bacteria called sulfate reducers use hydrogen to turn that sulfate into hydrogen sulfide. Besides the rotten-egg gas, farts can contain other sulfur-containing gases, including methanetriol and dimethyl sulfide, and the odor goes up and down with the hydrogen sulfide level. Foods high in sulfur, like proteins and cruciferous vegetables such as cauliflower, cabbage and broccoli, are usually behind foul-smelling gas.
Some carbohydrates are not digested or absorbed in the small intestine, and the share differs by food: 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; most foods with carbohydrates can cause gas, but fats and proteins cause little gas, and fat slows digestion, which gives food more time to ferment; the longer food stays in the large intestine, the more time bacteria have to ferment it, so more gas can build up. 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 of about 500 milliliters from one change in the menu. Food carbohydrates come as sugars, starches and fiber, and the stomach and small intestine cannot fully digest some of them.
Two things change how much gas a person has: how much air they swallow, and which foods they eat. Gassy foods are not only beans. Beans make more gas partly because of the sugars inside them: beans and other legumes are full of sugars people cannot digest, and soy flour made with less of those sugars produces less gas than ordinary soy; foods with fiber, like beans and cabbage, make more gas because the body cannot fully digest fiber, so bacteria in the large intestine do the job. Fruits such as apples, pears and prunes, and vegetables such as broccoli, onions, carrots and cucumbers, are all foods linked to extra gas. Raffinose is a complex sugar found in beans, cabbage, Brussels sprouts, broccoli, asparagus, other vegetables and whole grains. Almost anyone who eats large amounts of vegetables or fruits gets some extra gas.
Carbohydrates come in several kinds, and they differ in how much gas they make. 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. Certain carbohydrates called FODMAPs are poorly absorbed and rapidly fermented by bacteria in the intestines, which makes more gas. Milk sugar is called lactose, and fruit sugar is called fructose, and both can cause gas in some people. Sorbitol, mannitol and xylitol are sugar alcohols used in some sugar-free candies and gum, and people cannot digest them well; most starches, including potatoes, corn, noodles and wheat, produce gas as they are broken down in the large intestine, and rice is the one starch that does not.
Swallowed air has its own causes. Drinking through a straw is one of the everyday ways to swallow extra air, and so are chewing gum, sucking on hard candy, talking with your mouth full, eating or drinking too fast and drinking carbonated drinks. Swallowing lots of air can make gas build up in your digestive system; most belching comes from swallowed air, and that air most often never reaches the stomach and instead builds up in the esophagus. Some people burp a lot because they swallow too much air and release it before it reaches the stomach. Taking your time over a meal, and not gulping or talking while eating, means you swallow less air. Carbonated drinks and beer are made fizzy with carbon dioxide gas, and when you drink them the gas is released inside you and can come back up as burps.
Gas is a mix of gases: carbon dioxide, nitrogen, oxygen and hydrogen, and sometimes methane. 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. About one in three people make methane gas in the large intestine, and gas made in the large intestine can burn, because it has hydrogen and methane in it. Everybody has gas, and everybody farts and burps, and most of the gas that comes out of the bottom is made by bacteria breaking down food, not swallowed. Researchers are developing ingestible capsules that sense gases inside the gut as they travel. They give direct gas readings and are minimally invasive.
Words to know
- sphincters: ring-shaped muscles
- chromatograph: gas-sorting machine
- invasive: entering the body
- sulfate: sulfur compound
- raffinose: bean sugar
