There is always gas in your stomach and intestines, and everyone has it at all times, because swallowed air stays there until it is passed. That is why burping and passing gas are normal parts of how a body works rather than oddities of a few people. 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. 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. A little more air, or more food for the bacteria, means more gas.
Gas leaves the body at two different ends, and each end has its own name and its own story. Belching releases gas from the upper digestive tract, the esophagus and stomach. 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. It builds up in the esophagus, the tube that leads to the stomach. Swallowing air has its own medical name, aerophagia. Some people burp a lot because they swallow too much air and release it before it reaches the stomach. Most belching is caused by swallowing excess air, and belching during or after meals is normal.
People swallow extra air when they eat or drink too fast, talk while eating, chew gum, suck on hard candies or drink carbonated drinks. Gum and hard candy work through a simple mechanism: they 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, and when you drink them, the gas is released inside you and can come back up as burps. The reverse also holds, because taking your time over a meal, and not gulping or talking while eating, means you swallow less air, while swallowing more air may lead to more gas.
The two sources leave different fingerprints. 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: they 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. Swallowed air also mostly leaves by the mouth. Most of it comes back up as burps, and very little 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 instead of coming out.
Most of the gas that comes out of the bottom is made by bacteria in the large intestine as they break down food, not swallowed. Some carbohydrates, meaning sugars, starches and fiber, are not digested or absorbed in the small intestine. They move on to the large intestine, where bacteria break them down and make gas. The large intestine holds a large number of bacteria, plus some fungi and viruses, that together make up the gut microbiome. About three quarters of fart gas, 74 percent, is hydrogen, methane and carbon dioxide made by these bacteria as they ferment food. About one in three people also make methane gas in the large intestine, along with hydrogen and carbon dioxide. Not all gas is passed: a lot of it is absorbed into the blood or burped up before it ever leaves through the bottom.
Which foods matter most? Most foods with carbohydrates can cause gas, but fats and proteins cause little gas. Raffinose is a complex sugar found in beans, cabbage, Brussels sprouts, broccoli, asparagus, other vegetables and whole grains. Beans and other legumes have a high share of carbohydrates that people cannot digest, which is why they are so linked with gas. Most starches, including potatoes, corn, noodles and wheat, produce gas as they are broken down in the large intestine. Rice is the one starch that does not. Lactose is a natural sugar in milk and milk products, and many people cannot digest it well. Sorbitol, mannitol and xylitol are sugar alcohols used in some sugar-free candies and gum, and people cannot digest them well either. Almost anyone who eats large amounts of vegetables or fruits gets some extra gas. Fiber comes in two kinds, and they behave differently. 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.
How much gas is that? On average people pass about 600 to 700 milliliters of gas a day, which sits inside the usual range of half a liter to one and a half liters. The count of how often differs by source. As of November 2026, studies suggest an average of 8 to 14 times a day, and one hospital says 13 to 21. Another source says most people pass gas about 14 times a day. These numbers do not match, and the gap is part of the point: 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. People also pass gas more often around mealtimes and just after eating.
Then there is the smell. 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. Hydrogen sulfide is the waste of the microbes that help digest what the body cannot, and it is one reason gas can smell like rotten eggs. Foods high in sulfur, like proteins and cruciferous vegetables such as cauliflower, cabbage and broccoli, are usually behind foul-smelling gas. How strong a fart smells goes along with how much hydrogen sulfide is in it.
How do we know any of this? 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. Some research labs also analyze gas by counting how often people pass it and testing what it contains, and a mix rich in nitrogen points to swallowed air. A scientist who studies flatulence is called a flatologist.
Passing gas is rarely a sign of a medical issue, and healthy people make a lot of gas every day. Without a way out, gas trapped in the digestive system causes bloating and swelling of the belly, which is one reason a way out matters. Burping and passing gas are natural and common, and everybody does both, so a normal amount for one person is not the same as for another. 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. Taking your time over a meal, and not gulping or talking while eating, means you swallow less air.
Words to know
- aerophagia: swallowing air
- flatulence: passing gas
- raffinose: bean sugar
- chromatograph: gas-sorting machine
- flatologist: gas scientist
