What? No relevant XKCD?
this post was submitted on 17 Aug 2025
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A somewhat relevant joke:
A mathematician, a physicist, and an engineer are discussing how much their poops weigh. Math man says he weighs himself before and after poop. Physics guy says he uses water displacement to accurately measure poop mass. Engineer says “i just shit on the scale.”
I'd say you loose weight. Imagine opening a bottle of Coke, the gas comes out, and the whole surrounding is covered in Coke sprinkles. I suspect the same happens when you fart. Your scale may have high precision, but your farts don't.
Except if the gas applies pressure at the right angle it will create a force that makes you weigh more. Like air over a very sensitive scale.
Fart gas is warmer than the surrounding atmosphere, therefore less dense. Your digestive system is under very slight compression (10-20 mmHg gauge pressure according to the internet), which I would guess does not equate to enough pressure to be more significant than the temperature gradient. Fart gas is also less dense than air at a given pressure by a pretty significant margin (1.06 g/L compared with 1.20 g/L).
When you fart, you're releasing gas that is less dense than the atmosphere, which means you get slightly heavier. Think of yourself as a hot air balloon with a very tiny chamber, and when you release a 90 milliliter fart, you lose a little buoyancy and sink a little. You get heavier when you fart.
I haven't done the math, but I looked around on the internet at some numbers, and that's what I think. I also ignored this because it is clearly AI slop, which is a little upsetting.
You seem to be assuming that the volume is immediately replaced by the external atmosphere, which I doubt is valid, more likely that the volume of the person would decrease, at least temporarily. The weight of 1 Liter (assuming a massive fart) of air is 1.275g according to wolfram, so, using your density numbers above, 1.275 * 1.06/1.2 = 1.126g lighter. Measurable with a really good scale, if the 90ml fart volume is realistic (has to be more realistic I guess), that's ~.1g,
Think of yourself as a hot air balloon with a very tiny chamber, and when you release a 90 milliliter fart, you lose a little buoyancy and sink a little. You get heavier when you fart.
No, you get denser, but not heavier.
You seem to be assuming that the volume is immediately replaced by the external atmosphere, which I doubt is valid
No, I was assuming your volume decreases. I don't actually know that to be the case, but my assumption is that there isn't "extra" space inside a person, and so if you lose material from a part of your body that isn't encased in anything rigid your volume decreases slightly.
So maybe I did have my terminology wrong. When a hot air balloon deflates, it falls. The density went up, but that's not what's directly relevant. The weight went down, I guess, but the "number on the scale", weight minus buoyant force, went way way up, because it lost some lower-density volume that was making the whole thing float. The weight (in a strict physics sense) went down, sure. But the number on the scale (which I was incorrectly calling "weight") went up. Same thing for a farting person.
I feel like like I get denser everyday no matter I do.
How many litters of gas I'd need to pass to lose a kilogram on the scale? Let's consider I have indestructible guts that can be ever enlarging without rupture and we ignore the linear increase of pressure that would happen
This is like what would happen if Bill Nye needed to worry about sweeps.
Do you set fire on your fart immediately?
Then you probably have a lighter ;-)
I recall doing an experiment in high school where we weighed a balloon that was blown up vs deflated. The one that was blown up weighed more, but by barely anything.
Assuming gas composition and compression and moisture and temperature of breath are the same as a fart, then yes you lose weight.
But methane is lighter than air, and there are so many other variables that it's possible a fart would make you lighter. However that's because of boyancy, you are losing mass plain and simple.
What if I did a massive power shit while standing on the scales? Would I watch my weight go down in real time as it squirts out?
Blippi moment
Yes. And you may even see a temporary exaggerated decrease depending on the force it is expelled and direction aimed.
So you're saying with a large enough, violent enough poop i could fly?
For every action, there is an equal yet opposite reaction.
Power shit thrust
The scale shits back in the shitter?
Well I'd be aiming outwards so as to limit the amount of shit that lands on the scales to avoid skewing results.
Thank you. This is an essential consideration. Likewise, speaking scientifically, this kind of thing isn't helpful:
Conservation of mass would prevent any outcome other than the scale showing you getting lighter.
No household scale would be precise enough for this experiment though.
No, scales don't measure mass but weight, it is completely possible to lose weight and have the scale show a larger number because of buoyancy. For example, grab a helium balloon capable of holding up a 1kg mass mid-air and the scale would show 1kg less than when you release it. This is very simple to understand, how much would the scale show for a 1kg object tied to that balloon? 0 of course, the object is not even touching the scale, and a slightly heavier object would only be making that slight weight difference of pressure on the sensors, not the remaining 1kg.
So conservation of mass has nothing to do with the question here. It's all to do with whether farts are denser than air while inside your body.
whether farts are denser than air
They are.
Are you sure? It's made mostly of Nitrogen, Hydrogen and Methane, all of which would be lighter than air because they're at a higher temperature than the air outside.
I think that, theoretically, if someone's flatus contained an abnormally high amount of lighter-than-air gases, like hydrogen and methane, they might get very slightly heavier. Having a gas like that inside of you would, I think, provide a bit of a buoyant force lifting you away from the scale that would make your weight read lower, and releasing that gas would sort of drop your full weight onto the scale.
In practice, methane and hydrogen are only part of a fart, and other gases and such in the mix are heavier than air, so at best you might break even.
Probably a few caveats to that about temperature and pressure and such, and it's doubtful that anyone's gut produces enough of the right kinds of gas for that to happen.
That would assume the digestive system isn't under pressure, but I think it is slightly?
What if you had eaten something that created a bunch of hydrogen gas (and lived somehow)? Wouldn't standing on a precision scale at sea level, in a sea level atmospheric pressure and composition, show you getting heavier as you expel the hydrogen.
I'm not sure it would, unless the person's volume also changes considerably.
Buoyant force comes from a displaced volume of fluid (the outside air in this case)
My volume changes a lot. When I'm full of gas, I look like I'm pregnant.
Well your guts and skin and other tissues do have some elasticity, I suppose it is possible that a large gas bubble might be able to expand your abdomen slightly.
We're very much into spherical cows in a vacuum territory here. I don't think there's any way this would be realistically measurable,just fun to think about.
It would have to expand your abdomen slightly, assuming you don't have access to a fourth dimension.
You'd stay about the same, unless the gas you emit was substantially lighter or heavier than air. Methane should be lighter than air off the top of my head, but it's not the only gas in the mix, so it's hard to say without in-depth knowledge of the composition of farts, the subject's diet, and ambient air temperature too, since the fart is gonna be at body temp but the atmosphere won't.
Lighter, but just barely.
Mythbusters would have tested this. Good question.
the scale would momentarily say you got heavier because of the added force pushing down on it from air movement but you you actually get lighter and the scale will soon say that as well
Wouldn't it also lift you up with a similar amount of force? Similar to putting a fan on a sailboat to blow at the sails, the forces would cancel out somewhat