Ask how much air you can hold underwater and the honest answer keeps moving. Depth moves it. Whatever a cylinder gives you near the surface, plan on roughly a quarter of that at 100 feet. Nothing about the cylinder changed. The water did. That one relationship explains most of what frustrates people about gas planning in scuba diving, and it explains why a bigger tank stops helping sooner than you would guess.
So the useful question isn’t how much your cylinder holds. It’s how fast you empty it, and what you can do about that.
How Much Gas Is in a Standard Cylinder?
A cylinder is rated by the volume of gas it holds measured at the surface. The familiar aluminium 80 carries roughly 80 cubic feet at a full fill, which is where the name comes from. That figure is a volume, not a duration. How long it lasts depends on the diver and the depth, and on very little else.
Two divers can drop off the same boat with identical fills and surface twenty minutes apart. Capacity explains none of that gap. Breathing rate, trim, fitness and workload explain nearly all of it.
Why the Number on the Label Is Not Minutes
Ratings describe capacity under a fixed pressure. They say nothing about consumption. A gauge showing a full fill tells you what you have. It doesn’t tell you how long you have it, and that second number is the one you plan a dive around.
Material matters as well. steel and aluminium cylinders of the same rated volume swim very differently, and the one that trims better often costs you less gas than the one that holds more.
Depth Multiplies Every Breath You Take
A regulator delivers gas at whatever pressure surrounds you. Go deeper and each lungful holds more molecules, so it takes more from the cylinder. The effect is large and it is immediate. According to Scuba Diving Physiology in StatPearls at the National Library of Medicine, a diver must inhale 4 times the air at 100 feet than at the surface to deliver the same amount of oxygen to the tissues.
Pressure stacks up fast. Every 33 feet of seawater adds another atmosphere on top of the one already pressing down at the surface. At 33 feet you’re at 2 atmospheres and spending gas twice as quickly. At 99 feet you’re at 4.
Compare two real dives and it lands. Same diver, same cylinder, same relaxed breathing. The deep dive ends in half the time of the shallow one, and no amount of technique closes that gap.
How Do You Work Out Your Own Gas Rate?
Swim a flat, easy profile at a known depth for a set number of minutes. Note the pressure you used, then correct that figure back to the surface. The result is your surface air consumption rate. It shifts with fitness and workload, and it is the one number that makes a gas plan yours rather than a table’s.
From there the arithmetic is short. Divers Alert Network puts it plainly: simply multiply your SAC by the atmospheres of pressure at your planned depth. Do that before the dive rather than after it.
What to Do With the Number
Multiply for the bottom, then add the ascent and any stops. Then add a reserve you will not touch. Divers who plan this way stop being surprised by the gauge, and the habit carries over to every breathing system they ever dive.
The same arithmetic sizes an emergency supply, which is why working out how much bailout gas a dive really needs starts from that same personal figure.
A Bigger Cylinder Buys Less Than Divers Expect
More volume does help. Carry half again as much gas and you get half again as much time at any depth. The catch is what that extra volume costs you on the way there.
Bigger sets are heavier out of the water and draggier in it. Drag raises workload. Workload raises breathing rate. A diver wrestling a badly balanced rig can spend the extra gas paying for the extra gear.
Which is why trim and buoyancy usually pay better than capacity. Hovering costs almost nothing. Sculling and finning to hold a depth costs a great deal, every minute, for the whole dive.
If long, calm bottom times are the real goal, the next honest step is a conversation with people who dive this equipment weekly. Find an AP Diving dealer near you and ask what a normal dive on a closed circuit looks like in the water you actually dive.
What Changes on a Closed-Circuit Rebreather
Almost all of the arithmetic. On a closed circuit you still move the same volume in and out of your lungs, but you only consume the oxygen your body burns. That rate is metabolic, and depth does not multiply it. Small onboard cylinders then last for hours instead of minutes.
So the limits move somewhere else entirely. Scrubber life, decompression obligation and cold become the things that end a dive. The pressure gauge stops being the clock.
That shift is why so many divers coming from open-circuit scuba diving give up on bigger sets and change systems instead.
The hardware follows the same logic. The cylinders and valves a closed-circuit unit runs are small and specific, chosen to feed the loop rather than to store endurance.
None of that makes a rebreather easy. The gas maths gets simpler and everything else gets harder. Checklists get longer, maintenance stops being optional, and the failure modes are quieter than an empty cylinder. Silent Diving builds its range around AP Diving closed-circuit units and the consumables that keep them running, so the honest answer to a switching question is normally to train first and decide after.
Frequently Asked Questions
Why do two divers surface with very different pressures?
Because consumption is personal. Body size, fitness, thermal comfort, trim and task loading all move the rate, sometimes by a factor of two on the same profile. A diver who hovers effortlessly spends far less than one correcting depth every few seconds. Experience narrows that gap more reliably than equipment does.
Does nitrox make your gas last longer?
No. You breathe a nitrox fill at the same rate you breathe air, because consumption is driven by depth and workload rather than by mix. What nitrox buys is decompression margin and longer no-stop limits at some depths. On a dive where gas is the limiting factor, the mix changes nothing.
How much gas should you keep in reserve?
Enough for you and a buddy to reach the surface from the worst point of the dive, at a raised breathing rate, including any required stops. Many divers hold back a third of the fill as untouched contingency. The right figure comes from your own rate and the profile, not from a rule of thumb someone repeated on a boat.
Does cold water increase air consumption?
Usually, yes. Cold raises metabolic demand, thicker exposure suits add drag, and shivering adds work. Gloves and stiff suits make small tasks slower too, which means more time and more finning. Plan a colder dive on a higher rate than the one you measured on a warm reef.
Is slow, deep breathing a safe way to save gas?
Relaxed, full breathing is fine and efficient. Deliberately holding breaths between cycles is not, because it lets carbon dioxide build and raises the risk of headache, panic and worse. Efficiency should come from calm movement and good buoyancy. It should never come from restricting your own ventilation.
Do rebreather divers still need to know their gas rate?
Very much so. The loop may run for hours, but the bailout supply is open circuit and it gets sized with that same surface figure, at depth, under stress. A diver who has never measured the number cannot size an escape. It stays relevant long after the pressure gauge stops governing bottom time.
Your Next Step Toward Longer Dives
Measure your own rate first. It costs one easy dive and a notebook. If that number tells you depth is what ends your dives rather than discipline, the equipment conversation is worth having properly. Find an AP Diving dealer near you and ask about a try dive before you spend anything at all.