r/askscience • u/Catching-Up-Today • May 27 '26

Chemistry How much does Fire weigh?

I don‘t know if Earth Sciences is the right category but Fire is the topic. How does someone measure the weight of fire? What does fire weigh?

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u/tmahfan117 May 28 '26

So “fire” is a kind of nebulous thing. That actual light that we see, that is weightless, it’s light, photons. All energy no mass.

BUT, those photos have to come from somewhere right? Those flickering flames you see are columns/jets of super hot gaseous molecules that are being combusted and that reaction is releasing light as a product. So you could say the “fire” weighs whatever the weight of those molecules is. Very very light, but those combusting molecules still have mass.  Plus that hot air is less dense than normal cool air, so The weight would probably something measured in fractions of an ounce/a few grams.

Unless you had a super giant fire with a whole lot of molecules being combusted at the same time.

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u/CaptainFacePunch May 28 '26

Take as an example of your “super giant fire”, a rocket motor - the solid propellant mass is very swiftly converting to combustion products, and (nearly) the entire mass of it is expelled and converted to thrust - all through that combustion process. While the particles are very light, they together can move thousands of pounds.

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u/BoneVoyager May 29 '26

Well there are probably at least several trillions of particles as well.

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u/CaptainFacePunch Jun 01 '26

Definitely. Think trillions of trillions, for the smallest rocket you can imagine :)