Pressure and Your Body: Diving Physics Made Simple

Water pressure rises by about 1 bar for every 10 metres of seawater, and that squeezes or expands every pocket of air in and around your body. That one fact explains most of the rules on a scuba course: equalise early, never hold your breath, take the last few metres of every ascent slowly and keep adjusting your buoyancy.

This is general information to help the theory make sense, not medical advice: if you have ear, sinus or lung problems, talk to a doctor with diving knowledge before you dive.

Pressure With Depth: Absolute and Gauge

At sea level the air above you presses down with about 1 bar, often called one atmosphere. Water is far heavier than air, so only about 10 m of seawater adds another bar. Fresh water is slightly less dense than seawater (the US Navy Diving Manual gives the two densities), so in a lake it takes a little more, roughly 10.3 m, to add the same bar.

There are two ways to quote pressure:

- Absolute pressure is the total: the atmosphere plus the water above you. It is what your body feels and what the gas laws use. It is about 2 bar at 10 m, 3 bar at 20 m and 4 bar at 30 m. - Gauge pressure leaves the atmosphere out. Your cylinder's pressure gauge works this way, showing the difference between the air in the tank and the air around it, so a tank reading zero still holds air at 1 bar.

DAN's quick formula for seawater: absolute pressure in bar = depth in metres ÷ 10 + 1. At 18 m, that is 1.8 + 1 = 2.8 bar.

Boyle's Law: Why Air Spaces Shrink and Grow

Boyle's law says that at a constant temperature, the volume of a gas changes in inverse proportion to its absolute pressure. Double the pressure and the volume halves; halve the pressure and the volume doubles. Picture a flexible bag holding 1 litre of air at the surface, taken down without adding any air:

| Depth | Absolute pressure | Volume of the bag | Density of the air inside | |---|---|---|---| | 0 m | 1 bar | 1 L | 1x | | 10 m | 2 bar | 0.5 L | 2x | | 20 m | 3 bar | 0.33 L | 3x | | 30 m | 4 bar | 0.25 L | 4x |

The biggest changes happen near the surface. Going from the surface to 10 m halves the volume, while going from 20 m to 30 m only shrinks it from a third to a quarter. On the way up, air rising from 10 m to the surface doubles in volume, while air rising from 30 m to 20 m grows by only a third.

DAN uses the same maths to show why shallow water is risky for your lungs. Rising from about 4.5 m to the surface lets the air in your lungs expand by about 50%. Rising the same 4.5 m from 20 m expands it by only about 20%.

That is why ascents stay slow and controlled all the way up, and why the last few metres deserve the most care. The same expansion makes your wetsuit and BCD more buoyant as you near the surface: the US Navy Diving Manual warns that a diver can lose control of an ascent in roughly the last 9 m, where the pressure change is greatest. The slow finish also matters for nitrogen, which is why dives end with a safety stop (see our NDL guide).

Air Spaces: Ears, Sinuses and Mask

Your body is mostly water, which barely compresses. The trouble spots are rigid, air-filled spaces: your middle ears, your sinuses, the air in your mask, and occasionally a pocket of gas under a dental filling. As you descend, the air in them compresses, and unless you add air to match, the surrounding pressure pushes in. Divers call this a squeeze; the medical name is barotrauma.

Ears. Your middle ear is an air space behind the eardrum, joined to the back of your nose by the Eustachian tube. Equalising means easing a little air up that tube. DAN says ear problems make up nearly half the calls to its emergency hotline, and middle ear squeeze is the most common diving injury. It can start as shallow as 1 to 3 m, so DAN's advice is to equalise early (before your head goes under), equalise often, descend slowly and, if it hurts, rise a little and try again. Never force it: the US Navy warns that a forceful Valsalva (blowing hard against a pinched nose) can injure the inner ear. Our guide to equalisation covers the different techniques.

Sinuses. Your sinuses are air pockets in the bones of your face, linked to your nose by tiny openings. When those openings are clear, the sinuses usually equalise along with your ears. When a cold or allergy blocks them, you can get sharp pain in your forehead, behind your eyes or in your cheeks, or find blood in your mask after the dive. A blocked sinus can also hurt on the way up, as trapped air expands (a "reverse block"). DAN's advice: do not dive congested, and never underestimate sinus trouble.

Mask. Your mask is an air space too. Breathe a little air out through your nose as you descend; DAN suggests doing it every time you equalise your ears. Forget, and the mask works like a suction cup, which can leave bruising around your eyes and red patches in the whites of your eyes. Eye pain or any change in your vision afterwards needs a doctor promptly.

A few more rules. Never dive with earplugs, which trap air in your ear canal; the Navy warns they can also be pushed deeper. A tight hood needs to let some water in over your ears for the same reason. And tell your dentist you dive, since gas trapped under a poor filling can cause tooth squeeze.

If you have ear pain, hearing loss, ringing or dizziness after a dive, stop diving and see a doctor. DAN notes that any doctor can help, and an ear, nose and throat specialist is ideal.

Never Hold Your Breath

Your regulator delivers air at the pressure around you, so at depth your lungs are full of compressed air at their normal size. Hold your breath and rise, and that air expands with nowhere to go. DAN explains that once the tiny air sacs in your lungs (the alveoli) stretch past their limit, they tear. Air can then escape into the chest cavity (a collapsed lung), into the tissues around the heart and neck, or into the bloodstream, where bubbles can reach the brain. That last one, arterial gas embolism, is among the most serious diving emergencies.

It does not take much depth. DAN says a breath-hold ascent from as little as about 1.2 m, after breathing from a tank, may be enough to injure the lungs, which is why the rule applies in the pool too. The US Navy lists causes beyond deliberate breath-holding: panic, airways narrowed by conditions such as asthma or a bad chest infection, and, most commonly, running out of air and making an emergency ascent. That is why gas planning is part of lung safety, and why some lung conditions matter at your dive medical.

The rule itself is simple, and the Navy puts it plainly: never interrupt your breathing while ascending after breathing compressed air. Breathe normally and continuously, all the time.

If a diver is unwell after a rapid ascent, treat it as an emergency: call 000 first, then the DAN emergency hotline (1800 088 200 within Australia).

Buoyancy Changes: Wetsuit, BCD and Tank

Boyle's law also explains why your buoyancy keeps shifting. DAN sums it up: on the way down your wetsuit compresses, so you get heavier; as you breathe your tank down it gets lighter; on the way up the air in your wetsuit and BCD expands, so you get lighter.

- Wetsuit. Neoprene is full of tiny gas bubbles. They compress at depth, so the suit loses buoyancy (and warmth) as you go down and regains it on the way up. The Navy notes the deeper the dive, the bigger the effect. - BCD. Air in your BCD shrinks as you descend, so you add small puffs to stay neutral. On the way up it expands, so you let air out to stop the ascent speeding up. As DAN puts it, your BCD is not an elevator. - Tank. Air has weight. Using DAN's figure of about 1.29 grams per litre for air at surface pressure, a 12.2 L cylinder filled to 200 bar holds about 2,440 litres of air, which weighs roughly 3 kg. You finish the dive about 3 kg lighter than you started. PADI's test of good buoyancy is holding your depth at the safety stop with about 50 bar left, when the tank is at its lightest.

Our guides to perfect buoyancy and how many weights you need turn this into practical skills.

Denser Air and Narcosis

At depth you breathe denser air: about four times as dense at 30 m, DAN notes. Denser gas is harder to move through your airways and your regulator, so breathing takes more effort, and DAN says that by about 30 m the most air a diver can move in a minute falls to around half the surface value. Researchers Gavin Anthony and Simon Mitchell recommend keeping breathing gas at or below 5.2 grams per litre, which on air works out at about 31 m, with an absolute maximum of 6.2 grams per litre, about 39 m. The lesson for a new diver: at depth, slow down, avoid hard swimming and breathe slowly and deeply.

Nitrogen also has a narcotic effect that grows as its pressure rises. The US Navy describes it as a mix of euphoria and impaired judgement that varies widely between people and clears quickly when you ascend. Our nitrogen narcosis guide covers how to recognise it in yourself and your buddy.

Why Nitrogen Builds Up

Henry's law says the amount of a gas that dissolves in a liquid depends on the pressure of that gas. Your body is mostly water, so the deeper and longer you dive, the more nitrogen dissolves into your tissues, and on the way up it has to come back out. That is the reason for no-decompression limits, slow ascents and safety stops, all explained in our guide to the NDL. When it goes wrong, the result is decompression sickness.

Water and Heat

Water draws heat out of you far faster than air does. A 2021 paper in Diving and Hyperbaric Medicine notes that seawater conducts heat about 24 times better than air and, kilogram for kilogram, takes about four times as much energy to warm. The US Navy says a diver in just swimmers starts to get chilled once the water is below about 21°C, and a chilled diver cannot think clearly.

Depth makes it worse. Neoprene insulates less as it compresses: the same paper puts the loss at about half its thermal protection by 30 m. The Navy adds that you also lose more heat through breathing as the gas gets denser.

Thicker suits help, with a trade-off. A 2025 study of Tasmanian divers found that wetsuits of 7 mm or more slightly reduced lung volumes, most of all in female recreational divers. The authors thought the effect may be minor for fit, healthy divers but could matter more for less fit divers with medical conditions. Our wetsuit thickness guide matches suits to Australian water temperatures.

The Short Version

- Every 10 m of seawater adds about 1 bar, and volume changes most near the surface. - Equalise early and often, never force it, and do not dive congested. - Never hold your breath, above all while ascending. - Adjust your buoyancy in small amounts as depth and your tank change. - At depth, slow down, breathe easily and expect to get cold sooner.

Sources

- US Navy (NAVSEA): U.S. Navy Diving Manual, Revision 7 (2016), chapters 2, 3 and 7 (public-release copy hosted by Arizona State University) - DAN (Alert Diver): Estimating Your Air Consumption, Mike Ange, 2010 - DAN (Alert Diver): Lung Expansion Injuries, Daniel A. Nord, 2010 - DAN: Middle-Ear Barotrauma (MEBT) - DAN: Sinus Barotrauma - DAN: Mask Squeeze (Facial Barotrauma) - DAN: Ear Equalization Guidelines for Scuba Divers, 2020 - DAN (featured on the PADI blog): Buoyancy Control - DAN (Alert Diver): Performance Under Pressure, 2019 - DAN (Alert Diver): The Physiology of Compressed-Gas Diving, 2016 - DAN: Safety Tips Guide (call local emergency services first, then DAN) - DAN World: 24/7 Emergency Hotline numbers - Diving and Hyperbaric Medicine: Demers, Martin and Kartalov, Proof-of-concept for a segmented composite diving suit offering depth-independent thermal protection (2021) - Diving and Hyperbaric Medicine: Stevens and Smart, The influence of wetsuit thickness on lung volumes in scuba divers (2025) - PADI: 14 Tips to Improve Your Air Consumption

Header photo: A mushroom-shaped bubble of exhaled gas rising from a scuba diver, by Peter Southwood, CC BY-SA 4.0, via Wikimedia Commons.