What pressure is
Pressure is force spread over an area. At sea level the air above you presses down with about 1 bar, also written as 1 atmosphere (atm) or 14.7 pounds per square inch (psi). You don't notice it because your body is used to it and pushes back equally from the inside.
Water is about 800 times denser than air, so it adds pressure far faster. A column of seawater only 10 metres (33 feet) tall weighs as much as the entire atmosphere above it. That means every 10 m (33 ft) of seawater adds another bar.
Absolute and gauge pressure
Divers use two ways of counting pressure. Gauge pressure counts only the water above you, so it is zero at the surface. Absolute pressure adds the atmosphere on top, so it starts at 1 bar at the surface. Gas laws and decompression calculations always use absolute pressure, written as "bar" or "ata" (atmospheres absolute) in this wiki.
| Depth (seawater) | Gauge pressure | Absolute pressure | Gas volume compared with surface |
|---|---|---|---|
| Surface | 0 bar | 1 bar | 1 (full size) |
| 10 m / 33 ft | 1 bar | 2 bar | 1/2 |
| 20 m / 66 ft | 2 bar | 3 bar | 1/3 |
| 30 m / 99 ft | 3 bar | 4 bar | 1/4 |
| 40 m / 132 ft | 4 bar | 5 bar | 1/5 |
Fresh water and altitude
Fresh water is slightly less dense than seawater, so it takes about 10.3 m (34 ft) of fresh water to add one bar. The difference is small, but it matters for precise tables. Lakes at altitude bring a bigger change: the air pressure at the surface is already lower than 1 bar, so the same depth puts more relative strain on gas spaces and on decompression. Diving at altitude needs adjusted tables or a dive computer set for altitude, and specific training.
What pressure does to a diver
Rising pressure has four effects you'll meet throughout this wiki.
- Gas spaces shrink. Air in your ears, sinuses, mask and lungs is squeezed, as described in Boyle's law. That is why you must equalize your ears on the way down.
- You use more gas. Each breath at 20 m (66 ft) holds three times as many gas molecules as the same breath at the surface, so your tank lasts about a third as long.
- More gas dissolves into your body. Higher pressure pushes more nitrogen into your blood and tissues, which is the root cause of decompression sickness.
- Gases change their effects. Nitrogen becomes narcotic and oxygen eventually becomes toxic, as covered in nitrogen narcosis and oxygen toxicity.
The biggest changes are shallow
Pressure doubles between the surface and 10 m (33 ft), but it only rises by a quarter between 30 and 40 m. The relative change is largest near the surface, which is why ear squeezes usually happen in the first few metres of a descent and why buoyancy is hardest to control in shallow water. It is also why the final metres of an ascent need to be the slowest.
Frequently asked questions
How much pressure is there at 18 metres?
At 18 m (60 ft) of seawater the absolute pressure is about 2.8 bar: 1 bar of atmosphere plus 1.8 bar of water.
Why don't divers get crushed?
The body is mostly water, and water barely compresses. Only the air spaces feel the squeeze, and divers keep those equal to the surrounding pressure by breathing normally from the regulator and equalizing their ears and mask.
Is pressure the same in a lake and the sea?
Nearly. Fresh water adds slightly less pressure per metre, about 1 bar every 10.3 m (34 ft). Lakes at altitude start with lower air pressure at the surface, which matters more.