Boyle’s law in diving: why every air space changes with depth
In short
Boyle’s law says that at a constant temperature, the volume of a gas varies inversely with its absolute pressure: P1 × V1 = P2 × V2. In seawater, pressure doubles in the first 10 m (33 ft), halving any flexible air space. So divers equalize early, vent the BCD on the way up and never hold their breath.
Two kicks below the surface on your first descent, your ears already ache. You’re barely under, yet this is where pressure changes fastest in proportion. Boyle’s law describes that change, and most of the rules for descents and ascents follow from it.
What is Boyle’s law?
Boyle’s law states that for a fixed amount of gas at a constant temperature, pressure times volume stays the same: double the absolute pressure and the volume halves. NASA’s Glenn Research Center traces it to Robert Boyle’s work in the mid-1600s. Density does the opposite: air squeezed into half the space is twice as dense.
What does Boyle’s law mean for a diver?
Boyle’s law means every air space you carry changes size with depth. Water and most of your body barely compress; air does.
- Ears and sinuses: on the way down, you equalize, adding air through your Eustachian tubes. DAN notes that a difference of 0.35 bar (5 psi) may rupture the eardrum in some divers: roughly the pressure of just 3.5 m (11 ft) of seawater.
- Mask: breathe out gently through your nose as you descend, or the mask presses onto your face.
- Wetsuit and BCD: neoprene compresses as you descend, so you add air to your BCD, then vent it as it expands on the way up.
- Lungs: your regulator delivers air at the surrounding pressure, so your lungs stay full-size but hold denser air. Hold that breath while rising and it expands with nowhere to go.
Safety
Never hold your breath on scuba, above all while ascending. DAN reports that lung injuries can happen in a shallow swimming pool when a diver holds their breath on the way up: shallow water is no safer, as our dive theory myths explain. A suspected lung overexpansion injury is an emergency: oxygen and emergency services right away.
Try a question from the app
where the risk is greatest
Where is the risk of a lung overexpansion injury greatest?
Show the answer
Answer D. Volume changes most in shallow water, so a held breath is most dangerous there, even in a pool.
What is the Boyle’s law formula?
The Boyle’s law formula is P1 × V1 = P2 × V2, with both pressures absolute. To find the new volume: V2 = P1 × V1 ÷ P2.
Absolute pressure is the air above the water plus the water: 1 bar at the surface plus 1 bar per 10 m of seawater, or 1 atm plus 1 atm per 33 ft. NOAA gives 33 ft (10.06 m) per atmosphere and 1 atm is about 1.013 bar, so both rounded figures work. Gauges show gauge pressure, without the atmosphere: add it before using any gas law.
| Depth (seawater) | Absolute pressure | Volume of a flexible air space | Density of the air in it |
|---|---|---|---|
| Surface | 1 bar (1 ata) | 100% | ×1 |
| 10 m (33 ft) | 2 bar (2 ata) | 50% | ×2 |
| 20 m (66 ft) | 3 bar (3 ata) | 33% | ×3 |
| 30 m (99 ft) | 4 bar (4 ata) | 25% | ×4 |
Fresh water is about 3% lighter than seawater, so pressure builds slightly more slowly. Set your dive computer to fresh or salt water.
Drag the depth: Boyle’s law in action
Why do the biggest volume changes happen near the surface?
The biggest changes happen near the surface because Boyle’s law works on ratios of absolute pressure, and near the surface each meter (or foot) is a bigger share of the total. From 10 m (33 ft) to the surface, the volume doubles; from 30 m to 20 m (99 to 66 ft), it grows by only a third. So equalize most often early in the descent, and make the end of every ascent the slowest part.
Worked example
Same 3 m (10 ft), very different effect. How much does a flexible air space grow over the last 3 m (10 ft) of an ascent, compared with 3 m (10 ft) up from 30 m (99 ft)?
- Near the surface: 3 m is 1.3 bar, so V2 = 1.3 × V1 ÷ 1, 30% bigger. In feet, 10 ft is 10 ÷ 33 + 1 ≈ 1.3 ata: also about 30%.
- Deep: from 33 m (4.3 bar) to 30 m (4 bar), 4.3 ÷ 4 = 1.075, 7.5% bigger. From 109 ft (4.3 ata) to 99 ft: about 7.6%.
Over the same distance, the air space grows four times as much near the surface.
Try a question from the app
biggest change near the surface
Over which range does the volume of an air space change the most?
Show the answer
Answer A. Pressure doubles in the first 10 m (33 ft), so volume halves. Deeper, the same depth change is a smaller share of the total pressure.
How does Boyle’s law affect your BCD on the ascent?
Boyle’s law makes your BCD air expand as you rise, so you speed up unless you vent early, in small amounts.
Worked example
Air in your BCD on the way up. You hover at 30 m (99 ft) with 3 L (0.1 cu ft) of air in your BCD and start rising without venting.
- Metric: 4 bar × 3 L = 2 bar × V2, so 6 L at 10 m, and 4 × 3 ÷ 1 = 12 L at the surface.
- Imperial: 4 ata × 0.1 cu ft = 2 ata × V2, so 0.2 cu ft at 33 ft, and 0.4 cu ft at the surface.
- In seawater, each liter of air lifts about 1.03 kg (each cubic foot about 64 lb). From 30 m to 10 m (99 to 33 ft), you gain about 3 kg (6.4 lb) of lift. More on the buoyancy page.
What are the common Boyle’s law mistakes?
- Using gauge pressure. From 20 m to 10 m (66 to 33 ft), gauge pressure halves (2 to 1), but absolute pressure goes from 3 to 2: the volume grows by half, not double.
- Halving the depth. Only from 10 m (33 ft) to the surface does pressure halve.
- Applying the ratio backward. Deeper means smaller. If your volume grows on the way down, flip the ratio.
- Mixing units. Bar goes with meters, ata with feet: see metric vs imperial dive math.
Which dive course covers Boyle’s law?
In Octo, the Open Water Diver level teaches Boyle’s law as pressure and volume: ears, mask, lungs, BCD and air use. The Divemaster level returns to it with full calculations, alongside Charles’s, Dalton’s and Henry’s laws. Agencies organize theory differently, so check your own course materials. Test yourself with the Open Water practice questions, and keep every gas law at hand with the dive theory formulas cheat sheet.
Frequently asked questions
Does Boyle’s law apply to my tank?
Your tank is rigid, so its volume doesn’t change with depth. Boyle’s law matters for what comes out of it: your regulator delivers air at the surrounding pressure, so each breath at 30 m (99 ft) takes four times as much air as at the surface. More in air consumption and SAC rate.
Does Boyle’s law work the same in fresh water and at altitude?
The law is the same; the pressures change. Fresh water is lighter, so it takes slightly more depth to add the same pressure. At altitude, surface pressure is below 1 bar (1 atm), so the same depth means a bigger pressure ratio. Above 300 m (1,000 ft), altitude procedures apply: set your computer, or check that it adjusts on its own.
Why do I need more air in my BCD as I go deeper?
Because your wetsuit shrinks. Neoprene is full of tiny gas bubbles that Boyle’s law squeezes as the pressure rises, so you displace less water and lose lift. You make up for it with small bursts of air in your BCD, and vent that air on the way back up.
Is Boyle’s law the reason for the safety stop?
Partly. The 3-minute stop at 5 m (15 ft) gives your body time to release nitrogen (Henry’s law). It also helps you control the last part of the ascent, where Boyle’s law makes your BCD and lung volumes change fastest.
Sources
- NASA Glenn Research Center, Boyle’s Law (Beginner’s Guide to Aeronautics, updated March 2025)
- Georgia State University, HyperPhysics, Ideal Gas Law (Boyle’s law, gauge and absolute pressure)
- Georgia State University, HyperPhysics, Atmospheric Pressure
- NOAA National Ocean Service, How does pressure change with ocean depth?
- DAN, Ears & Diving, Chapter 1: Key Information
- Joseph Gessert, Shallow-Water Arterial Gas Embolisms, DAN Alert Diver (2021)
