Flatus—the proper medical term for gas emitting from the intestines—has been examined at length in comedy, on YouTube, and in dorm rooms around the world. A fart's volume, however, is not often addressed. Farts consist primarily of nitrogen, hydrogen, carbon dioxide, oxygen, and methane. These gases have mass, which means farts could theoretically be measured for volume. But has anyone ever bothered? And if so, how?
Yes. And, gently. In 1991, gastroenterologists from the Human Gastrointestinal Physiology and Nutrition Department of the Royal Hallamshire Hospital in Sheffield, England published a paper in the trade journal Gut that attempted to quantify toot size. Their methodology was simple: take 10 volunteers, feed them 200 grams of baked beans on top of their normal diet, and measure their flatulence over a 24-hour period via rectal catheters.
To confirm their collection protocol was up to snuff, subjects sat in a bath with the rectal catheter inserted—the line led to a laminated gas bag—and farted. Since no bubbles were visible, they concluded the catheter was sealed.
The physicians determined that the average adult produces a median 705 milliliters of gas (nearly 24 ounces, or two soda cans’ worth) every day. Men and women passed equal amounts; farts tended to be more robust following a meal.
The farters varied widely in individual output, with a range of 476 ml to 1491 ml among the subjects during the 24 hours. The researchers noted that a singular fart, regardless of time of day, gender, or body size, was between 33 to 125 ml, with a median of 90 ml. That’s the equivalent of roughly three ounces of comedy.
According to Matthew Bechtold, M.D. a gastroenterologist and associate professor of Clinical Medicine at the University of Missouri, fart volume can be highly variable depending on both the amount of swallowed air and the matter produced by bacteria in the colon. Simple carbohydrates that are incompletely digested feed these normal bacteria, which then produce gases. “The main carbohydrate responsible for flatulence is raffinose, a sugar commonly found in cabbage and broccoli, which is poorly digested,” he says.
The study noted that volunteers on low-fiber diets reduced most of the fermentation gases expelled, lowering their fart volume to 200 ml for an entire day.
The researchers did not indicate whether any solid matter would—or should—be considered when evaluating fart mass. According to Bechtold, no fecal material typically escapes during a gas pass. “The anus does well at keeping solid material in while letting the gas out,” he says.
With that settled, we posed another flatus-related query to Bechtold: If someone farted in cold weather with their pants down, could we visualize the fart similar to the way we see someone’s breath?
“Given the gas is contained in an environment of 98 degrees Fahrenheit just like the lungs, if gas is passed in a cold enough climate, it would likely be witnessed,” he says. “However, given that most patients are fully dressed with a barrier of pants between the anus and the outside environment, it generally goes unnoticed and diffuses rapidly in the air.”
Put another way: A fart is roughly the volume of an airport-approved travel bottle, and could be seen in cold weather if you were inclined to remove your pants in the name of science.