Water to Steam: Why Water Expands About 1,600 Times When It Boils

TL;DR: When water turns to steam it expands enormously. At normal atmospheric pressure, NIST data give boiling water a density of about 958 kilograms per cubic metre and the steam above it about 0.6, so the same mass of water takes up roughly 1,600 times more space as steam, or about 1,670 times if you start from cold water. Boiling also takes a lot of energy, about 2,256 kilojoules per kilogram, nearly seven times what it takes to heat the water from 20 C to boiling.

Last updated: 2 October 2026

Key takeaways

• Water to steam is one of the most dramatic everyday changes in volume. At atmospheric pressure the steam takes up about 1,600 times the space of the boiling water it came from.
• Starting from cold water at 20 C, the figure is about 1,670 times. That is where the commonly quoted round number of 1,700 comes from.
• A kilogram of steam at 100 C fills about 1.7 cubic metres, roughly the volume of a large wardrobe, from a single litre of water.
• Boiling takes far more energy than heating. Turning a kilogram of boiling water into steam needs about 2,256 kilojoules, nearly seven times the 335 kilojoules it took to heat it from 20 C.
• Real steam is invisible. The white cloud above a kettle is tiny droplets of water forming as the steam cools.
• The expansion explains why firefighters say never to throw water on a pan fire, why volcanoes can explode without new lava, and why every hot water cylinder must stop its water going above 100 C.

How much does water expand when it turns to steam?

When water turns to steam at normal atmospheric pressure, the same mass of water takes up roughly 1,600 times more space. The numbers come straight from the US National Institute of Standards and Technology. Its thermophysical property tables for water give water at its boiling point of 99.97 C a density of 958.37 kilograms per cubic metre, and the steam in equilibrium with it a density of just 0.59766. Divide one by the other and you get about 1,604.

Most people start with cold water, though. The same NIST table gives water at 20 C a density of 998.21 kilograms per cubic metre, which makes the full journey from a cold glass of water to steam at 100 C an expansion of about 1,670 times. Round that up and you have the figure of 1,700 that turns up in textbooks and fire safety talks.

That is the short answer to what happens when water turns to steam. It is also a good way to picture it: a single litre of water, boiled away completely, would fill about 1.7 cubic metres with steam at 100 C and normal pressure. That is roughly the space inside a large wardrobe, from the contents of a large bottle.

Why does steam take up so much room?

Because in liquid water the molecules are touching, and in steam they are not. In a glass of water, molecules jostle shoulder to shoulder, held loosely together by hydrogen bonds that constantly break and re-form. There is very little empty space between them, which is why liquid water is so hard to compress.

Boiling breaks that closeness. Each molecule that escapes into the steam leaves its neighbours behind and flies off on its own, colliding only occasionally with others. The molecules themselves are no bigger than before. What changes is the distance between them. At 100 C and atmospheric pressure, steam is mostly empty space, with the molecules spread out so thinly that a cubic metre of it weighs about 0.6 kilograms, a little more than half a bag of sugar.

The same molecules that are packed tightly as a liquid pack in an unusual open pattern as ice, which is why water also expands when it freezes. We wrote about that in why ice floats and why snowflakes have six sides. Freezing opens the molecules up a little. Boiling sets them free.

How much energy does it take to turn water to steam?

Much more than most people expect. Heating water and boiling it are two very different jobs, and the NIST tables put numbers on both.

Warming a kilogram of water from 20 C to its boiling point raises its enthalpy, the physicist’s measure of its heat content, from about 84 kilojoules to 419. That is about 335 kilojoules. Turning that kilogram of boiling water into steam at the same temperature raises it from 419 to about 2,676 kilojoules per kilogram, which is about 2,256 kilojoules more.

So boiling a kilogram of water away takes nearly seven times the energy it took to bring it to the boil. That extra energy, called the latent heat of vaporisation, goes entirely into pulling the molecules apart. None of it raises the temperature, which is why a pan of boiling water stays at the same temperature however hard the hob works, and why a kettle takes far longer to boil dry than to boil.

The energy does not vanish. When steam condenses back into water, it gives all that latent heat back. That is why steam can deliver far more heat than water at the same temperature, and why it has powered engines, turbines and heating systems for more than two centuries.

Why can you not see steam?

Because water vapour is a gas, and like the air around it, it is transparent. The Met Office’s explainer on what clouds are says it simply: as the sun heats water, it turns into an invisible gas called water vapour.

The white plume above a kettle or a cooling tower is not steam at all. It is water that has already turned back into liquid. As hot vapour leaves the spout and meets cooler air, it condenses into countless tiny droplets, which scatter light and look white. Look closely at a boiling kettle and you will see a clear gap just above the spout, where the steam is still hot enough to stay invisible, before the white cloud begins.

The same condensation builds real clouds. The Met Office describes how rising water vapour eventually becomes cool enough to turn back into water, forming the clouds we see. A kitchen kettle and a cumulus cloud are the same process at different scales, and we covered the sky version in how much a cloud weighs.

Does water always boil at 100 C?

No. It boils at 100 C only at normal atmospheric pressure. Change the pressure and the boiling point moves with it, because boiling happens when the vapour pressure of the water matches the pressure pushing down on it.

The NIST saturation tables show the shift clearly. At about 0.74 bar, roughly three quarters of normal atmospheric pressure, water boils at about 91 C. At about 1.84 bar, it boils at about 118 C. At 99.97 C, the table’s own figure, it boils at 1.01325 bar, which is standard atmospheric pressure.

That is why food cooks faster in a pressure cooker: the pressure inside raises the boiling point, so the water can get hotter before it turns to steam. It is also why water boils at a lower temperature high on a mountain, where the air pressure is lower.

It works in reverse too. Water that is under high pressure can be heated well above 100 C and stay liquid. If the pressure is suddenly released, some of that water flashes to steam almost instantly, expanding hundreds of times over as it goes. That is why hot water under pressure has to be treated with such respect.

Does steam keep expanding if you heat it further?

Yes, but far more gently. Once every drop has turned from water to steam, any extra heat raises the temperature of the steam itself, which engineers call superheated steam. From then on it behaves much like any other gas: hotter means a little bigger, and the growth is slow.

The same NIST isobar table shows how modest this second stage of steam expansion is. At atmospheric pressure, a kilogram of steam at the boiling point fills about 1.67 cubic metres. Heat it to 200 C and it fills about 2.14 cubic metres, only about 28% more. The big jump happens at the boiling point itself, at the moment water vapour first forms.

The energy tells the same story. Taking a kilogram of steam from 100 C to 200 C adds about 200 kilojoules, according to NIST. The latent heat needed to turn that kilogram of boiling water into steam in the first place was about 2,256 kilojoules, more than eleven times as much. Almost all the effort of turning water to steam goes into the change of state.

What happens when water meets something much hotter than 100 C?

It turns to steam so fast that it becomes explosive. The expansion that fills a kitchen with harmless mist over minutes can happen in a fraction of a second when water hits oil, metal or rock far above its boiling point.

In the kitchen. A pan of cooking oil can be much hotter than boiling water. The London Fire Brigade’s advice on pan fires is blunt: never throw water over a fire, as it could create a fireball. The water sinks into the hot oil, flashes to steam and throws burning oil into the air. The brigade’s advice is to leave the pan, turn off the heat only if it is safe to do so, get out, close the door and call 999.

In volcanoes. Some volcanic eruptions contain no new lava at all. The US National Park Service describes phreatic eruptions as eruptions driven by steam explosions, caused when groundwater is superheated by a magmatic source. Its example is the 2006 eruption of Fourpeaked Volcano in Alaska’s Katmai National Park, which sent an ash cloud 20,000 feet into the atmosphere.

In industry. The same physics is why hot water and steam systems are built with so many safety devices. A vessel of very hot water under pressure holds an enormous amount of stored energy, and releasing it suddenly releases the expansion with it.

Why must a hot water cylinder never go above 100 C?

Because stored water above its boiling point is a steam explosion waiting for a leak. Building regulations in England deal with this directly. Requirement G3(3) of Approved Document G says a hot water system with a hot water storage vessel must incorporate precautions to prevent the temperature of the stored water at any time exceeding 100 C, and to make sure any discharge from safety devices is safely carried to where it is visible but will not endanger people.

In practice that means more than a thermostat. The guidance describes non-self-resetting energy cut-outs, which disconnect the heat supply if the water overheats and stay off until someone investigates, and temperature relief valves, which discharge water safely if it gets too hot. Unvented cylinders, which run at mains pressure, typically carry both.

That discharge pipe, often a copper pipe ending over a drain or gully with a visible gap called a tundish, is one of the few places in a building where the physics of water to steam is written into the plumbing. If water ever comes out of it, the system is telling someone something has gone wrong.

Frequently asked questions

How much does water expand when it turns to steam?

At atmospheric pressure, steam takes up about 1,600 times the volume of the boiling water it came from, based on NIST density data. Measured from cold water at 20 C, the expansion is about 1,670 times, which is often rounded to 1,700.

Why is steam invisible?

Water vapour is a transparent gas. The white cloud above a kettle is made of tiny liquid droplets that form when the vapour cools in the air. Just above the spout, where the steam is still hot, there is often a clear gap you can see through.

How much energy does it take to boil water away?

About 2,256 kilojoules per kilogram at atmospheric pressure, according to NIST data. That is nearly seven times the 335 kilojoules needed to heat the same kilogram from 20 C to the boil.

Why does water boil at a lower temperature on a mountain?

Because the air pressure is lower. Water boils when its vapour pressure matches the pressure above it, so lower pressure means a lower boiling point. NIST data show water boiling at about 91 C at roughly three quarters of normal atmospheric pressure.

Why should you never pour water on a pan fire?

Because the oil is much hotter than boiling water. The water sinks, flashes into steam and expands violently, throwing burning oil out of the pan. The London Fire Brigade warns it could create a fireball and advises getting out and calling 999.

Is water vapour the same as steam?

Yes. Steam is water vapour, the gas that forms when water boils or evaporates, and like all water vapour it is invisible. In everyday speech people also call the white cloud above a kettle steam, but that cloud is made of tiny liquid droplets that have already condensed.

What is latent heat?

It is the energy a substance absorbs or releases when it changes state without changing temperature. For water to steam at atmospheric pressure, NIST data put it at about 2,256 kilojoules per kilogram, which is why boiling water stays at its boiling point until the last drop has gone.

One last thought

Every time a kettle clicks off, a small miracle of physics has just happened. A few hundred millilitres of water have started turning into a gas that wants to take up the space of a room, held back only by the lid and the cool air of the kitchen. Steam built the railways and still drives the turbines in many power stations, and it all starts with molecules letting go of each other.

We watch water all day, in pipes, tanks, rivers and sewers, including the hot water systems where temperature matters most. It still surprises us. For more short, sourced stories about water itself, read why rain smells, how waves break and why the ocean is blue.

By GP, Digital and IoT lead at AQUAIOT.