So asketh my 13yo this morning. :o
I honestly don’t know the answer. Logic tells me that I lose weight, but logic might not be the right tool for this question.
Farts are matter, matter has mass, and mass has weight. So by a strict definition of weight, you definitely lose it.
A laxer definition of weight might include buoyancy. You definitely lose some buoyancy when you fart, but it may or may not be greater than the amount of weight lost. Farts do contain some lighter-than-air gases such as methane and hydrogen, but they also contain heavier-than-air carbon dioxide and air itself, and while they’re in your body, they’re also under slightly increased pressure. On balance, I’d guess that fart-gas inside a body is usually slightly denser than air, and so you would lose less buoyancy than weight.
So you definitely lose mass, but your actual weight loss may vary, depending on the gaseous content of your flatus and its relative density compared with air.
And as I recall from numerous TMI threads on these boards, a mist of liquid and solid waste does fly out as well. And those are definitely more weight than any gas.
When I fart, I don’t know if I gain or loose weight but I tend to clear the room out. Everything that breathes air leaves quickly.
Yes, but you need to not be wearing any pants or underwear or the matter will be trapped there and thus your weight on the scale would not change.
I wonder if Astronauts in the ISS have moved through station on fart power?
This will bring peals of laughter to my little girl… thanks! 
Depends on your definition of weight. Weight is often defined strictly in terms of mass and gravitational force, making it irrelevant what atmosphere you’re standing in.
If you define weight as the reading on a scale, then it matters.
Who stirred the O2 tanks?
Gas has weight, and you’re losing gas, so you’re losing weight, however miniscule.
So I guess, the question is, when we say lose weight, do we mean net or gross. Because a helium balloon floating above a scale will ‘weigh’ less than that same balloon after I’ve poked it with a pin and all that’s happened is it’s lost mass.
The Freddy the Pig diet:
- Fart
- Change underwear
- Weight loss!
So I guess my dining room weighs more now than when I moved in.
So I googled “density of flatus”–I was going to ask here, but some times I get yelled at for not doing minimal research–and the top hit, would you believe it, was Typical Density of Flatulence. What kind of people would bother wondering about that?
Well, in a way… 13 year olds? 
Us?!? 
You just did the impossible, you made farts not funny.
Did he forget to mention that the way you smell farts is by the components of that mist entering your nose?
So in that respect are they like Ebola or not?