Atmospheres to Bar

Depth, Ambient Pressure and the Gas in Your Cylinder

Dive theory counts pressure in atmospheres absolute — ata — because that is the number the gas laws care about. Dive gear, on the other hand, is stamped in bar almost everywhere outside North America: the cylinder gauge, the compressor plate, the first-stage intermediate pressure, the inflator spec on a wing. Sooner or later the plan you worked out in ata has to be spoken in bar.

The factor: 1 atm = 1.01325 bar. A diver at 20 m of seawater sits at 3 ata, and 3 × 1.01325 gives 3.03975 bar of ambient pressure — three times the squeeze on every air space, from a wetsuit hood to a middle ear.

Four Pressures a Diver Juggles

Ambient pressure

What the water and the sky press on you together. It starts at 1 ata on the boat deck and climbs by roughly one atmosphere for every 10 m of seawater.

Breathing gas density

A regulator hands over gas at ambient pressure, so a lungful holds as many molecules as the ata figure. At 4 ata a breath contains four times what it would on the surface.

Cylinder contents

A 12 L cylinder filled to 200 bar is stored gas, not surroundings — about 2 400 litres of free gas waiting behind a valve, a completely separate number from the ata around you.

Intermediate pressure

The first stage tracks the water, holding its set margin — typically 9 to 10 bar — above whatever the ambient pressure happens to be, which is why it keeps working as you descend.

Running the Numbers Before You Splash

Gas planning is arithmetic done on dry land. Four moves cover the conversion side of it.

Put the depth figure in as atmospheres

Take the planned depth in metres, divide by 10 and add 1 — so 30 m becomes 4 — then type that into the left box. Bar appears alongside it straight away. A keypad comma works as well as a dot, and stray spaces are ignored.

Multiply your surface rate by the same figure

A surface air consumption of 20 litres a minute becomes 80 litres a minute at 4 ata. Against 2 400 litres in a 12 L cylinder at 200 bar, that is thirty minutes of gas before any reserve is set aside.

Lift the clean number for your plan sheet

Each field carries its own copy button and hands over the digits alone — no unit, no spacing — which drops straight into a dive-planning spreadsheet. Ctrl + C with the cursor in a field does the same job.

Turn it around for a bar-marked scale

Press the ↔ control and the page runs bar → atm instead, which is what you want when the figure came off European kit. The reverse multiplier is 0.986923267, so 6 bar works out at 5.9215 atm.

Nothing here replaces a dive computer or a table. Ambient pressure is only one input to decompression planning, and conversion arithmetic says nothing about no-stop time, ascent rate or gas mix limits.

Absolute Pressure Across the Recreational Range

Depth in metres and feet set against the pressure a diver is actually sitting in — first as the ata figure used in training, then converted to bar. Feet are rounded to whole numbers.

Depth (m)	Depth (ft)	Absolute pressure (ata)	Absolute pressure (bar)	Gas use vs surface
0 m	0 ft	1 ata	1.01325 bar	×1
10 m	33 ft	2 ata	2.0265 bar	×2
20 m	66 ft	3 ata	3.03975 bar	×3
30 m	98 ft	4 ata	4.053 bar	×4
40 m	131 ft	5 ata	5.06625 bar	×5
50 m	164 ft	6 ata	6.0795 bar	×6
60 m	197 ft	7 ata	7.09275 bar	×7

Notice how uneven the first ten metres are compared with the last ten. Dropping from the surface to 10 m doubles what the lungs handle; going from 50 m to 60 m adds only a sixth on top. That lopsidedness is exactly why the shallowest part of an ascent is the part that wants the slowest, most deliberate handling.

What the Page Gives a Gas Planner

Type into whichever box holds your number

Neither field is locked. Enter an ata figure on the left or a bar figure on the right and the other side follows, so a whole multi-level profile can be walked through without clearing anything.

Flip toward the equipment side

One press of ↔ reverses the direction, which is the way round you want it when the number came off a compressor plate or a fill-station chart rather than out of a dive plan.

Metres of water and psi in the same list

Searchable dropdowns on both sides carry all twenty-six pressure units in eight groups, so a hydrostatic head written in mH₂O or an American gauge in psi lands here too.

Digits a logbook can carry

Results run out to eight decimal places with a space between thousands, so 3.03975 stays intact instead of being rounded into a training figure you would have to defend later.

What Divers Ask About Pressure at Depth

Is 2 ata the same as 2 bar on a submersible pressure gauge?

No — the two count from different starting points. An ata figure counts up from a perfect vacuum, so 2 ata is 2.0265 bar of real, absolute pressure. Almost every mechanical gauge, the one on your cylinder included, is zeroed to the surface and reads gauge pressure, so the same condition shows as roughly 1.01 bar on that dial. Gas-law arithmetic — density, partial pressures, consumption — only works with the absolute figure.

Why is one atmosphere counted as 10 metres of seawater, and what changes in a lake?

It is a training convenience that happens to land very close. Seawater near 1 025 kg/m³ delivers one bar in about 9.95 m and a full atmosphere in about 10.08 m. Fresh water is lighter, so a lake needs roughly 10.20 m for a bar and 10.33 m for an atmosphere. Across a 30 m dive that gap is about a metre of equivalent depth — small, but it is one reason freshwater and altitude tables exist at all.

How much faster does a cylinder empty at 30 metres?

Four times as fast as on the surface, because 30 m is 4 ata and every breath carries four times the gas. Take a diver whose surface rate is 20 litres a minute: at depth that becomes 80 litres a minute, and a 12 L cylinder holding 2 400 litres at 200 bar covers thirty minutes with no reserve at all. Plan against the pressure you will be breathing at, not the pressure on the boat.

My cylinder reads 200 bar — how does that compare with the pressure around me?

They are separate quantities that happen to share a unit. At 40 m the water around you is at 5 ata, or 5.06625 bar; the gas inside the cylinder is at roughly forty times that. The cylinder reading tells you how much gas remains, while the ata figure tells you how quickly it will go. Both matter, and neither substitutes for the other.

Does nitrogen left over from an earlier dive change the pressure figures?

Not the ambient ones. Twenty metres is 3 ata whether it is the first dive of the trip or the fourth, because ambient pressure depends only on depth and the water above you. Residual nitrogen is a question of how loaded your tissues already are, which is what a surface interval, a repetitive group letter or a computer's tissue model tracks. Converting atm to bar describes the surroundings and says nothing about that loading.