Air Consumption by Depth Chart

Air consumption scales directly with ambient pressure. At 33 feet you breathe twice as much gas per minute as at the surface, at 66 feet three times, and at 99 feet four times, and none of that is because your breathing changes. Each breath simply holds more gas, because the regulator delivers it at ambient pressure and ambient pressure rises with depth. This is the single most useful fact a new diver can carry, and the chart below is that fact applied to a real cylinder.

Everything on this page comes from one relationship: ambient pressure in ata = depth in fsw / 33 + 1. Learn that and you can reconstruct every row here on a boat deck without a chart. A diver who knows the depths where the multiplier hits 2, 3 and 4 makes better decisions about turn pressure than one who has memorised a table of durations for somebody else's breathing rate.

How much does depth multiply your air consumption?

Read the multiplier column. It is nothing more than ambient pressure in atmospheres, rounded, and it applies to whatever your surface breathing rate happens to be. The last two columns show what that looks like for a diver with a respiratory minute volume of 0.5 cubic feet per minute, priced both in cubic feet and in psi off an aluminium 80.

Three rows are worth committing to memory because they are exact rather than approximate. 33 feet is 2.00 ata, so double. 66 feet is 3.00 ata, so triple. 99 feet is 4.00 ata, so quadruple. Everything between them interpolates smoothly, and if you can hold those three you can estimate any depth in your head to within a few percent.

The psi per minute column carries a warning with it. Those figures are true for an aluminium 80 and nothing else, because a psi is a different amount of gas in every cylinder. Move the same diver to a steel HP117 and the needle moves more slowly for identical breathing. That is the reason gas planning is done in cubic feet, and the per cylinder conversions are on the tank specifications chart.

How long does an aluminium 80 actually last?

A full aluminium 80 holds 77.4 cubic feet at 3,000 psi. Ending the dive on a 500 psi reserve leaves 64.5 cubic feet usable. Divide that by your consumption rate at depth and you get the duration below. Typical adult recreational respiratory minute volume runs 0.4 to 0.75 cubic feet per minute, which is roughly 11 to 21 litres per minute, so the four columns bracket almost every recreational diver.

Two comparisons in that table do most of the teaching. Read across the 99 foot row and the spread between an efficient diver and a heavy breather is 40 minutes against 22, on the same cylinder at the same depth. That difference is entirely skill and physiology, not equipment, and it is why buying a bigger cylinder is the wrong first response to short dives. Then read down the RMV 0.5 column: the same diver gets 65 minutes at 33 feet and 32 at 99. Depth is the variable with the most leverage over your dive time, by a wide margin.

Every figure assumes the whole dive happens at one depth, which no real dive does. A multilevel profile that touches 90 feet briefly and then spends most of its time at 40 will run far longer than the 90 foot row suggests. That is a reason to treat the table as a comparison tool rather than a planner, and to use the SAC rate calculator with your own logged dives instead of a generic rate.

How do you work out your own consumption rate?

From a dive you have already done. Take the pressure you used, the time you were down, and your average depth, then normalise to the surface.

SAC (psi/min) = (psi used / dive time) / ((average depth in fsw / 33) + 1)
RMV (cu ft/min) = SAC x (cylinder capacity / cylinder service pressure)

Work a real example through. You used 2000 psi over 40 minutes at an average depth of 50 feet. That is 50 psi per minute at the surface reading. Ambient pressure at 50 feet is 2.52 ata, so your SAC is 50 divided by 2.52, which is 19.9 psi per minute. Convert with the aluminium 80's 0.0258 cubic feet per psi and you get an RMV of 0.51 cubic feet per minute, comfortably inside the normal range.

RMV travels between cylinders and SAC does not. A psi per minute figure is only true for the cylinder it was measured in, so if you log your rate in psi and then dive a different cylinder, the number lies to you. Log the cubic feet figure once and it stays valid everywhere, including on a rental fleet abroad. Average it over several dives rather than trusting one, because a single cold, current heavy or anxious dive is not representative of you.

What does this mean for gas planning?

That the cylinder empties fastest exactly where an emergency is most expensive to solve. Rock bottom, the minimum gas reserve, is the amount two divers need to get from depth to the surface while sharing, with a safety stop, at an elevated breathing rate. It is built from four pieces: one minute at depth to solve the problem, an ascent at 30 feet per minute priced at the average depth of that ascent, three minutes at 15 feet, and the ascent from 15 feet to the surface, all for two divers at an elevated rate of around 1.0 cubic feet per minute each rather than a relaxed 0.5.

Worked at 100 feet in an aluminium 80 that comes to roughly 33 cubic feet, which is about 1,300 psi. That is the pressure at which the dive ends, not a number to swim past on the way to something interesting. On an out and back recreational dive you turn when you have used half of the usable gas, where usable means your start pressure minus that reserve. The gas planning calculator runs the whole sequence for your depth and your rate.

The rule of thirds, a third out, a third back and a third in reserve, is the overhead environment rule rather than the open water one. It exists because in a cave or inside a wreck you cannot ascend to solve a problem, so the reserve has to cover the whole exit. Explaining it is fine. Diving it requires specific training, and the technical diving explainer is where that conversation belongs.

How do you actually consume less gas?

In order of how much they return, and none of the first three involve buying anything.

Fix your weighting and your trim. An overweighted diver carries a large bubble of compensating air in the BCD, presents a bigger frontal area, and swims head up so that every fin kick pushes partly downward. Correcting this is routinely worth more than every other change combined, and the method is on the scuba weight chart.

Slow down. Drag rises steeply with speed, so swimming a little slower costs almost nothing in distance covered and saves a great deal of gas. The divers who last longest underwater are almost always the ones who look like they are barely moving.

Breathe slowly and completely. Short shallow breaths move air in and out of the dead space of the airway without exchanging much of it, which lets carbon dioxide build and creates the feeling of air hunger that makes divers breathe harder still. A long, complete exhale is the fix. Staying warm helps too, since a cold diver breathes more.

Once those are handled, monitoring becomes the useful upgrade. A tank pressure transmitter paired to a Shearwater Peregrine puts pressure and remaining time on the same screen as depth, and a console unit such as the Aqua Lung i300C does the same thing on a hose. Either way keep a mechanical submersible pressure gauge as backup, because a wireless link is one more thing that can drop. The air integrated computer roundup covers the options, and the air consumption guide covers the technique in depth.

Does any of this change in fresh water or at altitude?

Fresh water changes the depth, not the arithmetic. It takes 34 feet of fresh water to make one atmosphere against 33 feet of sea water, because fresh water is less dense, so at any given depth you are at very slightly lower ambient pressure and consuming very slightly less gas. The difference is around 3 percent, which is smaller than the variation in your own breathing between two dives, and most divers plan on the sea water figure everywhere and quietly take the difference as margin.

Altitude is a bigger effect and it runs the other way. At altitude the surface pressure is below one atmosphere, so the pressure ratio between the surface and a given depth is larger than it would be at sea level. That matters far more for decompression than for gas consumption, and it is why altitude diving has its own procedures, its own tables and a computer setting you have to remember to change. A dive at 8,000 feet of elevation is not the same dive as one at the coast, and treating it as one is a well understood way to get hurt.

Metric divers can run every figure here in litres and bar with the same structure. Ambient pressure in ata is depth in metres / 10 + 1, respiratory minute volume runs about 11 to 21 litres per minute for most adults, and cylinder capacity is quoted as a water volume in litres multiplied by the fill pressure in bar. A 12 litre cylinder at 200 bar holds 2,400 litres of free gas, which is close enough to an aluminium 80 to be a fair comparison. The multipliers do not change at all: 10 metres is still double, 20 metres is still triple, and 30 metres is still quadruple.

Related charts and tools

• SAC rate calculator, your own rate from a logged dive
• Gas planning calculator, rock bottom and turn pressure
• Tank specifications chart, cubic feet per psi by cylinder
• No-decompression limits table, the other limit on your dive
• Air consumption guide, how to breathe less without trying harder