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A row of tumble dryers with red laundry spilling out.

Clothes dryers

Everyone loves fresh, clean clothes—but how many of us enjoy the chore of getting them that way? Not many! Why? Because the process of cleaning clothes with water and detergent is inefficient, laborious, and time consuming. Clothes washing machines do a superb job of rinsing away the sweat and dirt our garments pick up each day, typically in less than an hour, but they have one mighty drawback: they give you back your clothes clean but soaking wet! Before you can put your t-shirt and jeans back on, you need to get rid of a huge amount of water, either by indoor or outdoor drying. What's the best way to do that? Let's look at the science and find out!

Photo: Industrial tumble dryers in a large laundry. Photo by Eric Cerami courtesy of US Army and DVIDS.

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  1. The science of drying clothes
  2. Can you dry clothes outside on cold days?
  3. What are the perfect conditions for drying clothes?
  4. How does a tumble dryer work?
  5. In summary...
  6. Find out more

The science of drying clothes

If you understand a little bit of the science behind drying your clothes, you'll find you can do your laundry a whole lot more quickly and economically. So what exactly is involved in turning wet clothes back into dry ones? In a word, evaporation: turning the liquid water in your clothes into a vapor (gas)—and then getting rid of it.

A row of socks hanging on a washing line.

Photo: Outdoor drying: When you hang socks outside to dry, you're using energy from the Sun and the wind to evaporate and disperse the water they contain. It's free to dry your clothes this way and you'll find it helps them last longer compared to using a tumble dryer. Since you're not using electricity to get things dry, it's better for the environment too.

How much water is in wet clothes?

Water is the world's most versatile cleaner, but it's amazing how much of it you can waste when you do the laundry. On its shortest and simplest setting, with only a half load of washing, my old clothes washing machine uses about 20 liters (5 gallons) of water; with multiple rinses, many machines will easily use double that much.

Most of that water is spun out at high speed (think centrifuge) and then drained away, but even the most efficient machines leave a significant amount of wetness lingering in your clothes. While writing an excellent book about how people can use energy more efficiently, physicist David MacKay weighed his laundry and discovered that a typical 4kg (8.8lb) load of dry washing emerged 2.2kg (4.9lb) heavier, even after vigorous spinning in a modern machine (read more in his discussion of home heating and cooling). So the price you pay for clean clothes is wet clothes that are at least 50 percent heavier than they were when you loaded them into the machine.

Before you can wear those clothes, or put them safely away in your cupboard, you have to get rid of that water. So the everyday chore of drying clothes is actually a more scientific kind of problem: how can we dispose of at least 2kg (4.9lb) of water as quickly and efficiently as possible? If you bear in mind that 1kg (2.2lb) of water is roughly 1 litre (2.1 US pints), you can picture the problem very vividly: for every load of wet washing, what you have to do is make the equivalent of at least two full liter bottles of water (just over four pints) literally vanish into thin air. That's actually quite a tall order—and it's probably rather more water than you imagine your clothes containing. Dry your clothes inside your home (without an electric dryer) and what you're doing is throwing about a quarter of a bucket of water into the air in extremely slow motion. Think about that the next time you hang your wet socks to dry on your bedroom radiator. Would you really want to hurl a quarter of a bucket of water in there?

The water in a typical load of washing is enough to fill two liter bottles.

Photo: Two liters of water: This is how much you'll find in your typical 4kg load of washing when you remove it from your clothes washing machine. It's what you have to get rid of before your clothes are properly dry. If you dry your clothes indoors on radiators or a clothes rack, this water will end up floating around inside your home. (Pen in front indicates scale.)

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How can you remove the water from your clothes?

Let's deliberately simplify...

The simplest way of getting rid of liquid water is to turn it into a vapor (broadly speaking, that means a gas produced from a liquid)—and the easiest way to do that is to heat it up. The molecules in a liquid are closer and more tightly bound together, move more slowly, and have less energy than the molecules in a gas. (This idea is part of the kinetic theory of matter—a way of understanding how solids, liquids, and gases behave by thinking about the molecules inside them buzzing about in constant motion.) If you want to turn a liquid into a gas, you need to put in quite a bit of energy so the liquid molecules can break apart, escape from the bulk of the liquid, and form a vapor above it. Putting heat into a liquid is an easy way to achieve this. Heat a liquid and you make the molecules, on average, more energetic so they have more chance to escape. Heat it enough and all the molecules will eventually evaporate—in theory, at least—leaving you with no liquid at all. So one way to dry wet clothes is to heat them up, turn the water they contain into steam, and then extract the steam so dry clothes are left behind. That's roughly how a tumble clothes dryer machine works, as we'll discover in a moment.

Illustration of drying a wet T-shirt: turning the cold water embedded in your clothes into hot water, and the hot water into steam (water vapor)

Artwork: Drying, in a nutshell, means turning the cold water embedded in your clothes into hot water, and the hot water into steam (water vapor).

Turning liquids into gases sounds like the stuff of chemistry labs, but it's exactly the sort of science we all dabble in every day. Whenever you boil a kettle, you convert cold liquid water into hot boiling water and—unless you turn the kettle off promptly—an astonishing amount of hot water vapor, commonly known as steam. Bear in mind that steam at a temperature of 100°C occupies roughly 1500 times as much room as liquid water and you'll understand why drying clothes (or cooking) can very quickly generate a massive amount of hot, humid air (saturated with water).

Producing steam from water requires a huge amount of energy, known as the latent heat of vaporization (you can read more about this in our article on states of matter). How much energy, exactly? Suppose the wet clothes from your washing machine contain 2kg (4.9lb) of water at 15°C (59°F). To get rid of that water, let's say you use an electric tumble dryer, which first heats the water from 15°C to 100°C (from 59°F to 212°F), using about 700kJ of energy, and then turns it into 2 kg of steam at the same temperature, using another 4500kJ of energy and making about 5200kJ (1.4 kilowatt hours) of energy in total. That's about as much as running an energy-saving lamp continually for 6 days and nights.

Electric dryers are expensive to run. That's partly due to the basic physics involved: it takes lots of energy to turn cold water into hot steam—and there's really no way around that. In reality, the energy you need is quite a bit more than I've estimated here because you're heating the clothes as well as the water, rotating them constantly in a drum, and losing energy all the time in various ways (through the walls of the dryer into the room outside). Natural gas dryers are a bit more efficient, with less energy wasted outside the drum.

In practice, much of what I've just said was theoretical rather than practical and deliberately quite a simplification. You don't actually need to heat all the water "locked" in your clothes and turn it into steam to remove it. Merely warming wet fabrics helps to release the moisture they contain and the steady airflow through a dryer will then remove the water like a breeze drying clothes hanging outdoors on a line. You could dry your clothes in a tumble dryer using cool air, if you wanted to, but it would take a lot longer. (Hotter air can hold more moisture, so a steady draught of warm air will carry water away more effectively.) Another important point is that different fabrics are made from different fibrous materials that hold and release moisture in different ways. So organic fibers such as wool (the inner part of which is hydrophilic—water loving) retain moisture much more than synthetic fibers, which therefore need less heating and dry more quickly. Some fabrics need to be heated and mechanically flexed so the fibers straighten out and release their moisture; with woollen clothes specifically marked "suitable for tumble drying," you'll be using lower drying temperatures and the gentle agitation of the drum will help to release water locked inside the fibers.

Can you dry clothes outside on cold days?

In a word, yes—but let me prove it to you, in theory and in practice.

In theory

The easiest way to get rid of water is to turn it from a liquid to a gas. If you look at the diagram below, you can see there are two ways to do this. One method is to use evaporation, in which the liquid turns directly to gas. You can also get from liquid to gas by making a solid first. For clothes drying, that means freezing your clothes (drying them on a winter day) and then letting the ice turn to gas (by sublimation). That's a very laborious way to dry your clothes but it does work—at least in theory!

Words we use for changing solids, liquids, and gases into one another.

The next diagram is what we call a phase diagram: it's a simple way of showing how a particular substance will be either a solid, liquid, or gas depending on the temperature and pressure. For water, the phase diagram is a little bit more complex than the one I've shown here, but let's assume this one is broadly correct.

Simplified phase diagram for water showing that ice, water, and steam can coexist at 0.01degC.

At high temperatures, you can see that water is going to be gas (steam) unless the pressure is high too; at low temperatures, water is generally going to be ice, unless the pressure is low. The diagram also shows us that you can heat solid ice or liquid water to make water vapor (gas) and squeeze (increase the pressure on) liquid water or water vapor to make ice.

Ice, water, and steam can all coexist at a certain temperature and pressure known as the triple point of water (shown by the green dot in the center). The magic temperature is 0.01°C, which proves (contrary to what many people believe) that you can happily dry clothes outside even in winter, providing the humidity is low (in other words, in a cool, dry wind). Assuming you start off at the same temperature, it's always going to require the same amount of energy to evaporate the water from your clothes, whichever drying method you use. On a cool day (when the rate of energy input from the Sun and wind is lower) it will obviously take much longer to get things dry. But a dry wind will still blow water away from your clothes, even if it does take all day.

Don't let the idea of "cold" put you off. From a scientific point of view, a "cold" day is only relatively cold: even when it's 0°C, that's still 273 degrees above absolute zero—the real definition of cold, the point where atoms and molecules stop moving—and there may well be enough energy around outside to dry your clothes. What's more, a "cold" day is not necessarily as cold as you think. If the sun is shining, there may be areas outside your home where the local temperature is significantly higher than the average—in a dry, sheltered courtyard, for example. Also, if the sun is shining directly onto your clothes, that will certainly evaporate water more quickly.

In practice

Now you might say "That's a load of baloney! I don't believe you!" But this is science and what you or I believe isn't what counts. Ultimately, what matters in science is what there's evidence for—so let's collect some and put the theory to the test.

On December 1, 2010, a day when a winter freeze was causing chaos throughout the UK, I decided to do an experiment with my washing. Where I live, there was no snow, but there was no sun either: it was just very cold (round about 0–1°C) and there was a bitterly cold, very dry easterly wind (according to the local weather forecast, 21–50km/h or 13–31mph). I did a small load of washing and weighed it when it emerged from the machine at roughly 5kg (11lb). I hung it outside for about four and a half hours, brought it in, and then put it back on the scales, discovering to my delight it now weighed only 3.5kg (7.7lb). The clothes weren't completely dry, but the cold wind had unquestionably removed a significant amount of water. After more drying indoors, I put the clothes back on the scales a third time and found they now weighed in at 3kg (6.6lb). Assuming they were completely dry at that point, we can see the wash added 2kg (4.4lb) to their weight, and outdoor drying removed 75 percent of the water. Pretty impressive for a winter's day!

Where does the water go?

If plants could talk, they'd tell you you don't have to heat water to its boiling point to make it evaporate. A plant can turn liquid water into water vapor at relatively low temperatures. When wind blows past trees, for example, water evaporates from their leaves and turns into cool water vapor that dissipates in the air. The more the wind blows, the more water plants lose by this process, which is known as transpiration, and the more you need to water them.

The reverse is true also. If the air is still, humid air can accumulate near the ground around plants, reducing the amount of water that they lose. Broadly speaking, blowing air past wet things means that water will evaporate from them more quickly, cooling them down in the process. And that's a very useful thing to know when it comes to drying your clothes. If you want them to dry properly, the water they contain doesn't just need to turn to a vapor; it has to be completely removed from the air around them. If water vapor lingers near your clothes, it'll not only hinder more liquid water from escaping, but some of the molecules in the vapor will also reenter your clothes and turn back into liquid, wetting them again! That's why hanging your clothes outside on damp days (when it's rained recently, or when there's low cloud or mist) is a waste of time: even if it's warm, the high level of humidity in the air will make it difficult for water to evaporate from your clothes and it's very unlikely they'll dry. They may even pick up moisture and get wetter!

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What are the perfect conditions for drying clothes?

To sum up, we've discovered that three things favor the evaporation of water from wet clothes:

  1. High temperatures—to increase the number of molecules that can turn from liquid to vapor.
  2. Air movements—to carry water away and prevent the air near your clothes from becoming saturated with vapor.
  3. Low humidity—so evaporation will continue steadily and water molecules won't return to your clothes from the air.

Now we've got the science nailed, let's compare the various different ways of drying clothes outdoors and indoors.

Outdoor drying

Outdoor drying (sometimes called air-drying) with a clothes line or rotary dryer has several big advantages: it's free, it uses no energy (so it's environmentally friendly), it generally leaves your clothes smelling fresh, and it means you don't make your home damp and cold by drying things inside. The drawbacks include the time taken to dry things (which can range from a few hours to a day or more), the chance of rain making your clothes wetter than they were when you hung them out, the risk of theft, and the possibility of air pollution making your clothes dirty again.

The science we've already considered tells us that the best conditions for outdoor drying will be warm, windy days when the humidity is relatively low. Since we want air to move around our clothes, it's best to dry them off the ground where the air moves faster (which also helps to lift them away from any ground-level moisture). Although hot, breezy, summer days are perfect for outdoor drying, you may find you can dry things outside for much of the year, if you keep science in mind (it depends to a large extent on the climate where you live). Water can turn from a liquid into a vapor at any temperature above freezing. In other words, providing the air isn't damp, you can theoretically dry clothes outside any day of the year. Living near the sea with the benefit of dry winter sea breezes, I can sometimes dry huge sheets outside even in the middle of winter (or get them half or two thirds dry at least).

Clothes on an outdoor washing line propped high off the ground to make them dry more quickly. Clothes on an outdoor rotary clothes dryer.

Photo: 1) If you're drying with a washing line, prop your clothes up high so they're above ground level moisture and benefit from higher wind speed. 2) A rotary clothes dryer like this uses space more efficiently than a traditional clothes line, but it might take longer to dry clothes if they're hung closer together and create higher humidity as a result. If you load a dryer like this properly, it should spin in the wind, which will help to dissipate water vapor. Whichever method you use, remember that you're trying to evaporate water from your clothes, so it will help to unpeg them after a couple of hours, turn them inside out or rotate them, and then rehang them.

In theory, you can even dry things outdoors in the very depths of winter. Instead of the liquid water evaporating and turning into a gas, it will cool down and turn to ice, which will then very slowly turn directly to a gas by the process of sublimation. If you want to dry clothes this way, you need to be incredibly patient and quite prepared to leave them outdoors for days until all the liquid water has turned to water vapor. If you let your clothes freeze solid and then bring them back indoors before they're dry, you're probably going to use more energy than if you'd simply dried them indoors to start with, because you'll have to heat them up to thaw them out (turning the ice back to water) and then dry them conventionally as well.

Indoor drying

If you remember only one thing from this article, make it this sentence: indoor drying may seem convenient, but it's a waste of money, harms the environment by wasting energy, slowly ruins your clothes (wrecking elastic and destroying things like t-shirt transfers), and risks damaging your home and your health by making your indoor environment damp and unpleasant. All these problems stem from two issues that follow directly from the science we've already explored.

Clothes inside the drum of a typical electric tumble dryer.

Photo: Inside a typical electric tumble dryer. Hot air enters through the holes at the back. Paddles mounted inside the drum (the gray line on the left) lift and tumble the clothes. Moist exhaust air exits through the lint filter at the bottom of the drum in front.

First, no matter how you dry clothes, you have to put in energy from somewhere to evaporate the water. Dry things outside and that energy comes for free from the Sun and the wind. Dry things on indoor radiators and the energy comes from your stove, gas boiler, or heating system. The laws of physics tell us that you cannot dry clothes for free indoors: the energy has to come from somewhere. You'll pay more to heat your home if you routinely dry things on radiators than if you don't. Even if your heating is on anyway, you'll need to have it on for longer (or turned up higher) to maintain the same temperature in your home and dry your clothes. If you dry clothes on a rack indoors, the energy needed to evaporate the water comes from the ambient (surrounding) air in your home—so your home is cooling down slightly to dry your clothes and costing you more that way. If you have no alternative to drying clothes indoors, you may find a dehumidifier is a good investment: by removing water vapor from the air, it makes your home healthier and can help to reduce heating costs.

A man unloads sheets from a large launderette dryer, viewed from the inside looking out.

Photo: Inside a typical gas tumble dryer. A tumble dryer in a laundry uses natural gas burners to dry your clothes inside a very large rotating drum. The burners, located at the bottom of the machine, create a steady updraft of hot rising air that floats up through hundreds of small holes in the drum. Paddles (vertical bars mounted around the inside of the drum) constantly toss and tumble your clothes so they repeatedly fall through the hot air. Photo by Walter M. Wayman courtesy of US Navy and Wikimedia Commons.

The second problem with indoor drying is that the water you evaporate has to go somewhere. With outdoor drying, this isn't a problem: the water simply dissipates in the air. Indoors, you'll find the steam or water vapor you create quickly appearing on your windows as condensation or (worse) as damp or mold on the walls. Remember how much water we're talking about: drying a typical load of 4kg (8.8lb) will add about 2 liters (about 4 US pints) of water to the inside of your home! Open the windows and you'll get rid of the water vapor, but you'll also lose all the heat energy the water contains (and some of the heat locked inside your building as well), so your home will cool down in the process—which isn't good in the middle of winter. If you dry with an electric tumble dryer, water vapor in your home isn't usually a problem: most dryers either vent the damp air out through a hose or condense it back to water that drips away down the drain. But the drawback is the huge amount of electricity they use.

Typical clothes drying times, as written on an electric tumble clothes dryer made by Bosch.

Photo: Quick dry: The biggest advantage of using an electric tumble clothes dryer is that it gets things dry much faster than almost any other drying method. These are typical drying times written on the case of a modern dryer made by the German company Bosch. According to this, even a big 6kg load would dry within a couple of hours, which is much faster than you'd manage outdoors in many countries.

How does a tumble dryer work?

We've seen from the science that the best drying conditions involve heat, air movement, and low humidity (or the constant removal of moisture). Clothes drying machines (also called tumble dryers) combine these things to dry clothes quickly and efficiently inside a large, rotating metal drum. Unfortunately, dryers like this cost a lot of money to run and they can shrink, damage, or slowly degrade your clothes (check washing labels on your clothes very carefully before putting them in a dryer like this).

Artwork showing the sequence of steps involved in drying clothes with an electric clothes drying machine.

The basic idea is to blow hot dry air into one side of the drum as it tumbles the clothes around and extract moist wet steam from another part of the drum at the same time. Step-by-step, here's how it works. (In this picture, we're looking from the side and the front of the machine is on the left.)

  1. The heart of the machine is a large metal drum with paddles around its inside rim (shown here as horizontal gray lines). In large machines, such as those in launderettes, the drum always rotates in the same direction. In smaller home machines, the drum rotates one way for maybe 30 seconds or so, then stops, then rotates the other way to stop your clothes bunching up.
  2. Cold air is drawn into the machine through an air intake. Often it's at the front of the machine to stop it getting dirty and dusty (as it would around the back).
  3. A fan sucks the air in and pulls it toward a heating element.
  4. The fan is powered by an electric motor (read more about how motors work).
  5. As cool air passes over the heating element, it's warmed and turned to hot dry air. A thermostat (not shown) turns the heating element on and off periodically to stop the machine overheating or cooking your clothes. When you select either the low or high temperature setting on your dryer, you're effectively altering the thermostat setting.
  6. Warm air from the heating element enters the drum, typically through large holes at the back. In launderette machines, the entire drum is full of small holes and hot air rises up from below. (The photo at the very top of this page shows a large laundry dryer like this.)
  7. The drum is rotated slowly by a belt connected to the electric motor, made from something like rubber. Often, as shown here, one electric motor drives both the drum and the fan.
  8. As the drum rotates, the paddles lift and tumble your wet clothes until they reach the top of the drum. Then gravity makes them fall back down through the hot, dry air. Dryers work most efficiently when the washing tumbles through the hot air this way. If you overload them, the washing just bunches up and rolls around in a big ball instead of tumbling and it takes much longer to dry.
  9. The air that leaves the dryer passes through a lint filter that catches dust and bits of fluff. Some dryers have a second (exhaust) fan to help extract the moist air. To avoid fires, it's essential to clean the lint filter in a dryer regularly (ideally, every time you used it).
  10. Exhausted air passes up through a vent hose either mounted permanently in the ceiling (as here) or temporarily poked through an open window. In some dryers, and most combined washer-dryer machines, the humid exhaust air is passed through a heat exchanger and condenser so the water is cooled and drained away and the heat it contains is captured and reused, making the whole process more efficient.

In summary...

So what's the best way to dry clothes?

  1. Air dry outside if you possibly can. It's better for your pocket, your clothes, your home, your health, and the planet.
  2. Choose your days carefully. Depending on the climate where you live, humidity may well be a more important factor than temperature. In other words, you might find you can dry things outside even on cold dry days if it's not too damp. Check out a weather website that gives a humidity forecast where you live. I find outdoor drying works best for humidities less than about 80 percent, really well for 75 percent or lower, and not at all if the humidity is over 90 percent.
  3. On dry winter days, humidity tends to fall to a minimum in the middle of the day. Hang things out mid-morning (after any mist or dew has disappeared) and bring them in before any evening dampness returns.
  4. The weather or time of year may prevent you drying things completely outdoors, but it's still well worth trying to get things half or two thirds dry that way. If you half dry outside and then tumble dry to finish, you'll still cut your dryer bills in half—and your clothes will last longer.
  5. If you must dry things indoors, the best way to do it is with an airing cupboard (the cupboard that surrounds your hot water tank, if you have one). If your home suffers from damp, routinely drying things indoors is a bad idea. If your kitchen and/or bathroom is dry or well-ventilated, consider drying clothes there to contain any moisture. If those rooms are constantly damp, that won't be possible.
  6. Using a launderette may seem expensive, but remember that there are hidden costs to drying things indoors at home: drying wet clothes in your house in winter will increase your heating bills. That's not an opinion, it follows directly from the laws of physics!
  7. Ventilate your home properly if you're routinely drying things indoors and consider buying a dehumidifier if damp becomes a problem.
  8. If you're going to buy a tumble dryer, check out its EnergyStar label (United States) or an equivalent energy ecolabel (in other countries). Remember that clothes dryers are just about the biggest energy-guzzlers in the home. If you buy the most energy efficient machine possible, you'll save yourself a great deal of money over the years and help the environment too.
  9. If disinfecting your clothes is important to you (if you're a healthcare worker washing a uniform, for example), remember that your hot tumbler dryer will do that more effectively than cool, outdoor, line drying. So you might want to tumble dry some of your washing while line drying, although you could, of course, pre-wash or soak things using liquid disinfectants.

Control panel of a Bosch electic tumble dryer that automatically stops when clothes are dry.

Photo: Energy-efficient drying: One reason tumble dryers cost so much to run is that most people "over-dry" their clothes. On older dryers, you select an arbitrary drying time and keep the dryer running until the clothes are dry. But that might mean the dryer is running much longer than it needs to. Modern dryers like this one, also made by Bosch, automatically stop when the clothes are dry. So instead of selecting a drying time, you select the type of clothes and how dry you want them—and let the dryer do the rest.

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There are hundreds of patents for all kinds of clothes dryers; here's a very small selection:

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