What Does Fire Weigh? Does fire have mass?


As a firefighter, you think a lot about how fire spreads, but probably not so much about what it weighs. Indeed, fire is not weightless, so it must weigh something. Exactly how much does fire weigh?

Fire weighs 0.3 kilograms a cubic meter if the air is at sea level and is 1.3 kilograms per cubic meter as well as if the gas from the flames is 1/4th less than the air density. 

If you’re interested in learning more about how much fire weighs, then you’ve come to the right place. Although measuring the weight and mass of fire isn’t easily achievable (given that it’s fire and extremely hazardous to your health), there’s still something to learn ahead. Keep reading! 

Does Fire Have Weight?

If you’re in a burning building, you probably have a million concerns floating through your mind. 

You worry about being burned by the flames, getting smoke inhalation damage by breathing in the dark smoke or being crushed by fallen debris and structures.

Fortunately, being crushed by fire is not very likely. Fire does indeed have weight, but nothing significant. 

We should take a moment to preface that mathematicians and physicists have argued for quite a while about how to determine the weight of fire. As we touched on in the intro, fire is extremely volatile, so it’s not an easily measured or quantified substance.

Fire is comprised of nitrogen, oxygen, water vapor, carbon dioxide, and plasma. It’s also a lot of very hot air.

You can thus use the ideal gas law or the general gas equation to determine that a gas’s density is proportional to its temperature. 

So, if a fire is burning and the gases are 1/4th as dense as the air and we know that the sea-level air is 1.3 kilograms a cubic meter, we can deduce that fire weighs 0.3 kilograms a cubic meter. 

Does Fire Have Mass?

Mass is a measurement of a physical body’s amount of matter. 

Once you know the mass of something or someone, you can use that information to determine the strength of the body’s gravitational attraction as well as its rate of inertia.

The next question, after pondering the weight of fire, is always this. Does fire have mass? 

More than likely, yes, it does. After all, fire contains hot gas, so it does have mass.

How would we determine what that mass is? Ah, now you’re asking a tricky question.

You would have to put fire into a container and then seal that container. More so, the fire couldn’t yet be burning for a long time.

Why not? You’d have to determine how much fuel the fire has in its beginning stages compared to the amount of gas. The longer the fire burns, the more impossible that is to do.

As the fire continued burning, you’d then have to determine how much the outside flame material weighs (remember everything that an average fire is made of) as well as the measurements of the flames. 

Finally, as the fire naturally petered out, you’d have to assess how much the oxidized products and fuel weighed.

Only then could you quantify the mass of fire!

Even still, doing so would require mathematically determining the amount of gas in the flame and what the gas’ mass was.

At current, the measurement options are far too challenging to quantify.

Fire can’t burn in an enclosed container since it needs oxygen to grow. Without oxygen, the fire would end quickly, possibly before you could measure its beginning stages versus middle stages versus end stages.

Even if you repeated the experiment, it wouldn’t work. You could maybe increase the size of the container so it has more oxygen, but the fire might last only marginally longer. 

Plus, working with fire is dangerous. 

Do you understand then why this has been unachievable? Maybe someday technology will advance to the point where a mathematician or physicist (or both) can use computer modeling to determine how much mass fire has.

Unless and until that happens, we can’t say for sure what the mass of fire is, only that it has mass. 

Does Fire Increase Weight?

Going back to what we mentioned in the first section, you cannot be crushed by fire. It’s simply not that heavy. It can burn you and choke you with its thick smoke, but it cannot crush you.

What about the falling objects and debris that so often accompany a burning building? 

Let’s say, for example, that a ceiling panel or a column was alight. Does the addition of the fire make the panel or column weigh more?

In other words, does fire increase the weight of an object that’s actively burning?

This is definitely a fascinating question, so let’s explore it a bit, shall we?

The mass-energy equivalence principle, which was created by Albert Einstein, is the classic E = mc2 formula. In that formula, c stands for the speed of light while m is for mass and E is for energy.

Einstein’s formula posits that if you warm up an object, its mass should be greater than it was before being heated. 

However, an object on fire is losing mass, and the smoke generated from the burning object is indicative of that. 

Thus, we’d say that fire does not increase the weight of an object. Instead, the object is actively withering away as it burns, becoming less and less as the flames grow higher. 

It’s certainly interesting to sit down and think about how much fire weighs and what its mass is. Indeed, fire does weigh something, although it isn’t much. We know that fire has mass too, although how much mass it is, we cannot say yet. 

We hope this article gave you something fun to think about!

Mike Pertz

I’m Mike, I’m a full time firefighter/paramedic and the founder of FirefighterNOW. I’m also a columnist for FireRescue1. If you’re reading this blog my guess is you are interested in the fire service. There's information on fitness, gear, interviews, tests and more. I hope you find what you're looking for.

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