Boyle's Law in Freediving: Pressure and Depth Made Simple
The Boyle's law freediving courses teach, made simple: how pressure rises with depth, what it does to the air in your lungs, ears and mask, plus a depth table.
Boyle’s law says that if temperature stays the same, the volume of a gas changes in inverse proportion to the pressure on it: double the pressure and the volume halves. This is the Boyle’s law freediving courses teach early on, and in practice it means the air in your lungs, ears, sinuses and mask shrinks as you descend and expands again as you come up. In seawater, pressure rises by about one atmosphere every 10 metres, so your lung air halves between the surface and 10 m, and the biggest changes happen closest to the surface.
That last point surprises people, and it explains a lot: why equalising is hardest in the first few metres, why the bottom of a 2-metre pool can hurt your ears, and why blackouts tend to happen near the surface rather than at depth.
Pressure and depth: one atmosphere every 10 metres
At sea level, the air above you presses down with a pressure of about one atmosphere, roughly 1 bar. (Strictly, 1 atmosphere is 1.013 bar, but divers use the two interchangeably.) Water is far denser than air, and a column of about 10 metres of seawater weighs as much as the whole atmosphere above it.
So the total pressure on you at any depth is air pressure plus water pressure:
- Surface: 1 bar
- 10 m: 2 bar
- 20 m: 3 bar
- 30 m: 4 bar
This total is called absolute pressure, sometimes written as ATA (atmospheres absolute). Fresh water is slightly less dense, so in a pool or lake it takes about 10.3 metres to add one atmosphere. The difference is small enough to ignore for most purposes.
Boyle’s law in plain words
The formula is P1 × V1 = P2 × V2: pressure multiplied by volume stays constant. Take 6 litres of air at the surface, at 1 bar. At 10 m the pressure is 2 bar, so 1 × 6 = 2 × V, and V = 3 litres.
Your body is mostly water, which barely compresses, so muscles, blood and bones don’t shrink in any meaningful way. Boyle’s law matters only for the spaces in and around you that hold gas:
- your lungs and airways
- the middle ear behind each eardrum
- your sinuses
- the air inside your mask
- gas in your stomach and intestines
- the tiny gas bubbles inside wetsuit neoprene
Depth, pressure and lung volume table
This table assumes 6 litres of air in your lungs at the surface, roughly a full breath for an average adult man. Your own litres will differ, but the fractions are the same for everyone.
| Depth (seawater) | Absolute pressure | Gas volume vs surface | 6 L of air becomes |
|---|---|---|---|
| 0 m | 1 bar | 1 (100%) | 6.0 L |
| 5 m | 1.5 bar | 2/3 (67%) | 4.0 L |
| 10 m | 2 bar | 1/2 (50%) | 3.0 L |
| 20 m | 3 bar | 1/3 (33%) | 2.0 L |
| 30 m | 4 bar | 1/4 (25%) | 1.5 L |
| 40 m | 5 bar | 1/5 (20%) | 1.2 L |
| 60 m | 7 bar | 1/7 (14%) | 0.86 L |
| 100 m | 11 bar | 1/11 (9%) | 0.55 L |
Why the first 10 metres are the hardest for equalisation
Look at what happens to gas volume over each 10-metre stretch:
- 0 to 10 m: volume halves (down 50%)
- 10 to 20 m: down by a third
- 20 to 30 m: down by a quarter
- 30 to 40 m: down by a fifth
The same depth change squeezes air much harder near the surface. Your middle ear is a small air space mostly surrounded by bone, and the only part that can flex is the eardrum. Close to the surface, each metre compresses that air by a large fraction, so the eardrum reaches the end of its stretch within a metre or two. Deeper down, the same metre is a smaller fraction, and you can go further between equalisations.
This is why instructors tell you to equalise early and often. It’s common to equalise gently at the surface before the duck dive, then every metre or so through the first several metres, before any discomfort appears. It’s also why your ears can hurt at the bottom of an ordinary 2-metre pool, where the air has already been squeezed by about a sixth. If you’re still working on the technique, our Frenzel guide breaks it down step by step.
Your mask follows the same rule
The air in your mask halves by 10 m as well. If you don’t add air, the mask presses into your face and pulls on the tissue around your eyes, which is mask squeeze. The fix is to breathe a little air out through your nose into the mask as you descend. It’s one reason freediving masks are low volume: less air to replace. See mask squeeze and sinus squeeze for prevention.
Running out of air to equalise: where mouthfill comes in
Equalising uses air from your lungs, moved up into your mouth and nose. As your lungs compress, that supply shrinks, and at some point it becomes hard to move air up from the chest at all. For many divers this point comes somewhere in the 20 to 40 metre range on a full breath, depending on lung size, flexibility and technique. Divers who go deeper take a reserve of air into the mouth at a shallower depth and use it for the rest of the descent, a technique explained in our mouthfill guide.
Buoyancy: why you float at the top and sink at depth
Your lungs and wetsuit act like built-in floats, and both lose volume as you descend. At the surface most people float. Somewhere on the way down you reach a depth where you neither float nor sink, called neutral buoyancy. Below that you become negative and start to sink, which freedivers use for freefall. A thick wetsuit makes this shift bigger, because the gas bubbles in its neoprene compress too.
Coming back up: expansion and the oxygen drop
On the ascent, everything reverses. Lung air expands back towards its starting volume, mask air expands and bubbles out around the skirt, and the air in your middle ears usually vents by itself.
Because a freediver only ever has surface air in their lungs, it can only return to its original volume, so the lung overexpansion injuries scuba divers worry about don’t apply. That changes completely if you take a breath from a scuba regulator at depth and hold it on the way up. Never do that.
The more serious issue on ascent is oxygen. Each gas in your lungs has a partial pressure, its share of the total. At depth, the surrounding pressure pushes the oxygen partial pressure up, which helps your blood stay loaded even as you use oxygen. As you rise, the pressure drops, fastest in the last 10 metres where it halves, and the oxygen partial pressure in your lungs falls with it. If you’ve used a lot of oxygen, it can fall below the level needed to stay conscious just as you approach the surface. This is the physics behind shallow water blackout.
A related effect involves nitrogen, which dissolves into your tissues in greater amounts under pressure. For a few recreational dives it isn’t a concern, but many repeated dives to depth with short rests can cause problems; see decompression sickness in freediving.
Safety: Pressure changes fastest near the surface, which is exactly where blackouts tend to happen. Your buddy should watch you all the way up and for at least 30 seconds after you surface. Never freedive alone, and never breathe from a scuba tank during a freedive.
Try Boyle’s law at your desk with a syringe
You can feel this physics with a plastic syringe from the chemist, no needle required.
- Pull the plunger back to the 10 ml mark.
- Seal the tip firmly with your fingertip.
- Push the plunger to 5 ml. That’s the surface to 10 m.
- Push it to about 3.3 ml. That’s 10 to 20 m. Notice how little the plunger moved this time, and how much harder you’re pushing.
- If your finger can hold the seal, push to 2.5 ml for 30 m.
The plunger travels 5 ml for the first 10 metres and well under 2 ml for the next 10. That shrinking movement, and the growing force behind it, is what your ears, mask and lungs go through on every dive. An online community for Indian freedivers is on its way here at freediving.in, so stay tuned.
Frequently asked questions
How much pressure is there at 10 metres underwater?
In seawater, about 2 bar absolute: 1 bar from the atmosphere and roughly 1 bar from the 10 metres of water above you. That is double the pressure at the surface, so any air space in your body or mask is squeezed to half its surface volume unless you add air to it, which is what you do when you equalise your ears and mask.
Does Boyle's law apply in a swimming pool?
Yes. The law is the same in fresh water; the only difference is that fresh water is slightly less dense than seawater, so it takes about 10.3 metres rather than 10 to add one atmosphere. At the bottom of a 2-metre pool, air spaces are already compressed by roughly a sixth, which is enough to make unequalised ears hurt.
Can water pressure crush the human body?
Not in the way films suggest. Your body is mostly water, which hardly compresses, so pressure mainly affects air spaces: lungs, ears, sinuses and mask. Those are managed by equalising and by the body's own adaptations such as blood shift. Elite divers in deep disciplines have gone well beyond 100 metres, though only after years of careful, supervised training.
What is the difference between absolute and gauge pressure?
Gauge pressure measures the water alone, so it reads zero at the surface. Absolute pressure adds the atmosphere on top, so it reads about 1 bar at the surface and 2 bar at 10 metres in seawater. Boyle's law only works with absolute pressure, which is why freediving depth and volume tables use it.
