Does air actually weigh anything? How heavy is the air in your room?
― A small room's air weighs about 28 kg, roughly as much as a young child
We use "air" as a byword for emptiness. But air really does have weight. A single litre comes to only about 1.2 grams, but fill a whole room with it and the picture changes. The air in an ordinary small room weighs around 28 kilograms. There's a clear reason you never feel that weight, though.
It's moving day. Every piece of furniture has been carried out, and you're standing in the empty room. The floor and walls are bare — it looks like there's nothing left inside.
But there's still something in that room you haven't moved out: the air filling it, wall to wall. If you could somehow bag it all up and put it on a scale, how far would the needle swing?
Most people would guess "basically zero." Do the actual sum, though, and the answer comes out about as heavy as picking up a small child.
Two reasons you never notice the weight
A litre of air weighs about 1.2 grams — roughly 1/800th as much as the same volume of water. But a room holds over 20,000 litres of it. Add up tiny weights tens of thousands of times and you get tens of kilograms.
Air pushes with equal force from the sides and below, not just from above. The air and water inside our bodies push back just as hard. Because the push and the push-back balance out, we never feel it as weight.
We'll start by estimating the weight of the air in a room, then move on to the much bigger story of the air stacked above your head.
A room's air gets heavier in proportion to its size
In an ordinary room at around 20°C, one cubic metre of air (a cube one metre on each side) is said to weigh about 1.2 kilograms. Multiply that by the room's volume and you get the weight of the air inside it.
Take a ceiling height of 2.4 metres: a small room of six tatami mats comes to about 23 cubic metres, so its air weighs about 28 kg — close to the average weight of a nine-year-old child.
Try the slider in Figure 1 to change the size of the room. A 20-tatami living room holds about 93 kg of air, easily more than an adult's body weight. A classroom (8 m by 8 m, 3 m high) comes to about 230 kg.
A 10-tonne column of air sits on your head
The air in a room is only a tiny slice of all the air there is. From the ground up to the top of the sky, air is stacked dozens of kilometres high. The column of air sitting on just one square metre of ground is said to weigh about 10 tonnes.
The left side of Figure 2 shows that column of air. It weighs about the same as the column of water on the right, which is 10 metres tall. The force of this weight pressing on the ground is what we call "air pressure" — familiar from weather forecasts. One standard atmosphere is about 1013 hectopascals.
So why aren't we crushed? The answer is the second card above: air pushes with the same force from every direction, not just from above. And the water and air inside our bodies push back outward with equal force — the same reason a fish can swim without feeling the weight of the water around it.
Air expands when it warms, so the weight packed into the same volume drops. Per cubic metre, that's roughly 1.29 kg at 0°C versus about 1.15 kg at 35°C. Even for the same six-tatami room, that's a difference of about 3 kg of air between the depths of winter and summer. This same difference is what lets a hot-air balloon float.
The total weight of the air blanketing the Earth is estimated at around 5,000 trillion tonnes — an almost unimaginable amount. Yet compared with the weight of the Earth itself, it doesn't even reach one millionth. To the planet, the atmosphere is a far thinner skin than an apple's peel.
Summary
A litre of air weighs only about 1.2 grams, but the air in a room comes to tens of kilograms, and the column of air above your head weighs about 10 tonnes per square metre. You never feel that weight because air pushes from every direction at once, and your body pushes back with equal force.
Air isn't light — it's just balanced.
Even an empty room is filled with the weight of a child.
To see what changes when the same thing happens underwater, read Why aren't deep-sea creatures crushed by all that water pressure? We've also worked out the weight of water floating in the sky in How much does a single cloud weigh?
- Fill a glass to the brim with water and lay a postcard or piece of thick card over the top. Do this over a sink.
- Holding the card in place with your hand, turn the glass upside down, then gently let go. The card won't fall, and the water won't spill.
- Next, put a straw in water, seal the top end with your finger, and lift it out. The water stays inside the straw until you lift your finger.
Both happen because the air pushing up from below is a stronger force than the water's weight pushing down. It's visible proof that air "isn't just pushing from above."
Want to go deeper? ― Terms, formulas, and how this connects to the textbooksWe've marked which level each part belongs to, from junior-high science to university-level specialist courses
- JHSCovered in junior-high-school science
- HSCovered in high-school physics or chemistry
- HS+High-school enrichment content, or textbook sidebar material
- Univ.Not covered in high school — university-level specialist content (atmospheric dynamics, atmospheric physics)
- ResearchNot yet settled even at university level — an active research question
JHSTerms: this phenomenon has names
- Density: weight (mass) per unit volume. Air comes to about 1.2 kg per cubic metre; water, about 1000 kg.
- Atmospheric pressure (air pressure): the force per unit area exerted by the weight of air. At ground level, roughly 1013 hectopascals.
- Pressure acts in every direction: inside air or water, the pushing force bears equally in every direction — up, down, left, right.
JHSHSCheck it with the formula: how heavy is a six-tatami room's air?
Weight is found from "density × volume." Work out the room's volume, then multiply by the density of air. At the end, we'll also work backwards from air pressure to find the weight of the column of air above your head.
| In symbols | m ＝ ρ × V (for a column: m ＝ p × A ÷ g) |
| In words | mass of air ＝ density of air × volume of room |
| Where it comes from | It's just the definition of density (mass per volume). The column formula comes from balancing the weight of air against the force pressing on the ground (pressure × area). |
| m | mass of air (kg) |
| ρ | density of air (kg/m³). About 1.2 at 20°C, 1 atm |
| V | volume of room (m³) |
| p | ground-level air pressure (Pa). About 101300 |
| g | gravitational acceleration (m/s²). About 9.8 |
| Area of 1 tatami mat (standard real-estate convention) | 1.62 m² |
| Ceiling height | 2.4 m |
| Density of air (20°C, 1 atm) | about 1.2 kg/m³ |
| Ground-level air pressure | about 101300 Pa |
| Floor area of 6-mat room | 6 × 1.62 ＝ 9.72 m² |
| Volume of room | 9.72 × 2.4 ≒ 23.3 m³ |
| Mass of air (the headline figure) | 23.3 × 1.2 ≒ 28 kg |
| Classroom volume (8 m × 8 m floor) | 64 × 3 ＝ 192 m³ |
| Mass of classroom air | 192 × 1.2 ≒ 230 kg |
| Air column resting on 1 m² | 101300 ÷ 9.8 ≒ 10337 kg |
About 28 kg for a six-tatami room, about 230 kg for a classroom, and about 10 tonnes of air resting on every square metre of ground. The air in a room is just a sliver cut from the very bottom of that column overhead.
HSHS+What determines the density of air?
HSThe density of air can be derived from the "ideal gas law" taught in high-school chemistry. Density rises with higher pressure and falls with higher temperature. That's why air is lighter on a hot day and heavier on a cold one.
HS+In 1643, Torricelli stood a tube filled with mercury upside down and showed that the mercury settled at about 76 centimetres. This is considered the discovery of atmospheric pressure. Mercury is about 13.6 times denser than water, so the equivalent water column would be about 10 metres — the same figure as the water column in Figure 2.
Univ.Why air gets thinner higher up
Each layer of the air column is compressed by the weight of everything above it. This balance is called "hydrostatic equilibrium." Assuming constant temperature, pressure falls off exponentially with height, dropping to roughly a third for every 8 kilometres of altitude gained. This length is called the "scale height." It's why the column in Figure 2 thins out toward the top.
📖 For the derivation and further reading: Atmospheric Pressure (Wikipedia, Japanese) ／ Evangelista Torricelli (Wikipedia, Japanese)
ResearchWhat's still not fully understood
- Fine fluctuations in the atmosphere's total weight The total mass of the atmosphere is thought to vary slightly by season, depending on how much water vapour it holds. Research continues into measuring that variation precisely.
- Air escaping into space At the very top of the atmosphere, light gases like hydrogen slowly leak away into space. How that rate changes with solar activity is still being observed.
- Indoor air and well-being Rather than the "weight" of a room's air, researchers are studying how carbon dioxide levels and humidity affect concentration. The findings aren't fully settled yet.
In other words, even this article describes things "as currently understood." The figures given assume 20°C and 1 atmosphere, and can shift by a few percent with weather or altitude.
How this connects to the textbooks, by level
| Level | Subject / unit | Where in this article |
|---|---|---|
| JHS | Science Year 1 "Density," Year 2 "Atmospheric pressure" | Weight of a room's air, what air pressure really is |
| HS | Chemistry "Ideal gas law" | Density difference between hot and cold days |
| HS+ | Physics "Fluid pressure" (enrichment) | Torricelli's experiment, correspondence with 10 m of water |
| Univ. | Atmospheric dynamics / atmospheric physics | Hydrostatic equilibrium, scale height |
| Research | Atmospheric science / indoor environmental science | Fluctuations in total atmospheric mass, atmospheric escape |
| ― | Connections to daily life | Weather-forecast air pressure, hot-air balloons, the glass experiment |
- Wikipedia, "Atmospheric Pressure" (大気圧)
- National Astronomical Observatory of Japan (国立天文台), ed., Chronological Scientific Tables (理科年表), Maruzen Publishing (density of air, standard atmosphere)
- Ogura Yoshimitsu (小倉義光), General Meteorology (一般気象学), University of Tokyo Press
- Trenberth, K. E. & Smith, L. (2005) The Mass of the Atmosphere: A Constraint on Global Analyses. Journal of Climate, 18, 864–875.
- Wikipedia, "Evangelista Torricelli" (エヴァンジェリスタ・トリチェリ)
※This article is a general-audience science explainer. The figures given are approximations meant to illustrate the underlying mechanism. If you try the glass experiment, do it over a sink and use a glass that isn't prone to shattering.