Scuba Tank Specifications Chart
An aluminium 80 scuba cylinder holds 77.4 cubic feet of air at a 3,000 psi service pressure, which is about 2.58 cubic feet per 100 psi, not the 80 cubic feet its name implies. A steel HP100 holds 100 cubic feet at 3,442 psi and a steel LP85 holds 85 cubic feet at 2,400 psi.
A scuba cylinder is described by two published numbers: its capacity in cubic feet and its service pressure in psi. Everything else a diver cares about, including how much gas is left at a given pressure, how long that gas lasts at depth, and how much the cylinder's buoyancy changes over a dive, is arithmetic on those two figures. The chart below runs that arithmetic for the cylinders you will actually encounter.
The first thing to fix is the naming. An aluminium 80 does not hold 80 cubic feet. It holds 77.4 at 3,000 psi, and the shortfall matters because gas planning is done in cubic feet. Cylinder names in the United States are approximate marketing labels, and the figure on the spec sheet is the one to plan with. The same is true in reverse for a steel LP85, which is called low pressure because its service pressure is 2,400 psi rather than because it holds less.
What are the specifications of common scuba tanks?
Capacity and service pressure are the published figures. Cubic feet per 100 psi, usable gas to a 500 psi reserve, and the buoyancy swing from full to empty are all derived from them below.
| Cylinder | Material | Capacity | Service pressure | Metric | Per 100 psi | Usable to 500 psi | Buoyancy swing |
|---|---|---|---|---|---|---|---|
| Aluminium 80 (Luxfer S80) | Aluminium | 77.4 cu ft | 3000 psi | 207 bar | 2.58 cu ft | 64.5 cu ft | 6.2 lb |
| Aluminium 63 | Aluminium | 63.4 cu ft | 3000 psi | 207 bar | 2.11 cu ft | 52.8 cu ft | 5.1 lb |
| Aluminium 40 (pony or stage) | Aluminium | 40 cu ft | 3000 psi | 207 bar | 1.33 cu ft | 33.3 cu ft | 3.2 lb |
| Aluminium 100 | Aluminium | 99.5 cu ft | 3300 psi | 228 bar | 3.02 cu ft | 84.4 cu ft | 8.0 lb |
| Steel HP100 | Steel | 100 cu ft | 3442 psi | 237 bar | 2.91 cu ft | 85.5 cu ft | 8.1 lb |
| Steel HP117 | Steel | 117 cu ft | 3442 psi | 237 bar | 3.40 cu ft | 100.0 cu ft | 9.4 lb |
| Steel LP85 | Steel | 85 cu ft | 2400 psi | 165 bar | 3.54 cu ft | 67.3 cu ft | 6.9 lb |
How long does each cylinder last at depth?
Duration is capacity divided by consumption, and consumption scales with ambient pressure. The table below takes the usable gas from the chart above, assumes a respiratory minute volume of 0.6 cubic feet per minute, which is a mid range recreational figure, and prices it at three depths. It assumes the entire dive happens at that depth, which no real dive does, so read it as a comparison between cylinders rather than as a dive plan.
| Cylinder | Usable gas | At 30 ft | At 60 ft | At 100 ft |
|---|---|---|---|---|
| Aluminium 80 (Luxfer S80) | 64.5 cu ft | 56 min | 38 min | 27 min |
| Aluminium 63 | 52.8 cu ft | 46 min | 31 min | 22 min |
| Aluminium 40 (pony or stage) | 33.3 cu ft | 29 min | 20 min | 14 min |
| Aluminium 100 | 84.4 cu ft | 74 min | 50 min | 35 min |
| Steel HP100 | 85.5 cu ft | 75 min | 51 min | 35 min |
| Steel HP117 | 100.0 cu ft | 87 min | 59 min | 41 min |
| Steel LP85 | 67.3 cu ft | 59 min | 40 min | 28 min |
The spread is the point. A standard aluminium 80 gives roughly 38 minutes at 60 feet on those assumptions, while a steel HP117 gives about 59. That is not a small difference, and it is why divers with a high breathing rate, or divers who want to match a buddy with a low one, eventually stop renting the house cylinder. It is equally why a bigger cylinder is the wrong first fix for a short dive: improving your breathing rate is free, and the SAC rate calculator tells you where you actually stand.
Notice also that the 40 cubic foot aluminium is not a primary cylinder for anybody. Its usable gas at 60 feet is around 20 minutes, and its real job is as a pony bottle or a stage cylinder, carried in addition to a primary. Carrying a redundant gas supply is a trained skill with its own failure modes, and the cylinder is the least important part of learning it.
Why does an aluminium tank float when it empties?
Because gas has weight, and you breathe it away. Air weighs roughly 0.0807 lb per cubic foot, so the 77.4 cubic feet inside a full aluminium 80 weighs about 6.2 lb. Breathe it down and the cylinder becomes 6.2 lb more buoyant than it was on the boat. An aluminium 80 runs about 4 lb positive at 500 psi, which is the specific reason weight checks are done at the end of a dive rather than the beginning.
A diver weighted to be neutral with a full cylinder is 4 lb light at the safety stop, at exactly the moment when being light matters most, because that is where a slow controlled ascent turns into a fast uncontrolled one. Steel cylinders behave differently: they stay negative throughout, so the swing still exists but never crosses zero, and a steel diver carries less lead on the belt to compensate. The full method is on the scuba weight chart, and the aluminium versus steel comparison covers the trade in detail.
One consequence catches travelling divers out. If you have dialled in your weighting on steel cylinders at home and then dive a rental fleet of aluminium 80s on holiday, you need more lead than you are used to, and you will only notice at the end of the first dive. Assume you will need a weight check on the first dive of any trip where the cylinders are not the ones you own.
How do you read remaining gas as cubic feet rather than psi?
Divide capacity by service pressure to get cubic feet per psi, which for an aluminium 80 is 0.0258, or 2.58 cubic feet per 100 psi. From there, 1,000 psi in an aluminium 80 is about 25.8 cubic feet, and 1,000 psi in a steel HP117 is about 34 cubic feet. Those are meaningfully different amounts of gas behind the same needle position, which is the whole reason gas planning is done in cubic feet.
The same fact explains why a SAC rate in psi per minute does not travel between cylinders while an RMV in cubic feet per minute does. A diver who measures 20 psi per minute in an aluminium 80 and then dives a steel HP117 will find the needle moving more slowly and might conclude they have improved, when nothing has changed except the size of the container. Convert to cubic feet once and the confusion disappears.
Whatever else you carry, keep a mechanical way to read pressure. A submersible pressure gauge cannot run out of battery and cannot lose a wireless pairing, which is why it remains the backup even for divers running a tank pressure transmitter on the wrist. The gas planning calculator works out the reserve you should be turning the dive on.
What inspection and testing does a cylinder need?
A scuba cylinder is a pressure vessel and is regulated as one. In the United States the schedule is a visual inspection annually and a hydrostatic test every 5 years, and fill stations enforce both by simply refusing to fill a cylinder that is out of date.
| Check | Interval | What it covers |
|---|---|---|
| Visual inspection (VIP) | Annually | Interior and exterior checked for corrosion, pitting, cracking and contamination. A sticker is applied and most fill stations refuse a cylinder without a current one |
| Hydrostatic test | Every 5 years | The cylinder is pressurised in a water jacket to measure permanent expansion. A pass is stamped into the crown with the date |
| Eddy current test | With the annual visual, on older aluminium | An additional check of the neck threads on aluminium cylinders made from 6351 alloy, which is no longer used for new cylinders |
| Valve service | Typically with the visual inspection | Burst disc, o-rings and valve seat. A cylinder is only as sound as the valve on top of it |
| Oxygen cleaning | As required by the fill station | Needed before partial pressure blending of nitrox at many shops. Ask before you assume a cylinder can take a rich fill |
Those recurring costs are the honest argument against owning cylinders early. A cylinder is cheap to buy and never free to keep: annual visuals, five yearly hydro tests, valve service, storage space, and the fact that it cannot be flown anywhere. Most divers are better served by renting for the first few years and spending the money on a computer, a mask that seals and exposure protection that fits, which is the order the gear buying order guide lays out.
When you do own one, the boring maintenance is what keeps it in service. Never drain a cylinder completely, because a small positive pressure keeps moist air out and moist air inside an aluminium cylinder is how internal corrosion starts. Rinse the valve and the cylinder neck after salt water. Store it upright and secured, since a cylinder falling over is a genuine hazard on a hard floor. And replace worn tank bands before they slip, because a cylinder sliding out of a BCD on a boat deck is one of the more alarming failures in diving. The gear maintenance guide covers the rest of the routine.
Related charts and tools
- Aluminium versus steel cylinders, buoyancy, cost and travel
- Air consumption by depth, how fast the cylinder empties
- SAC rate calculator, your own consumption in cubic feet per minute
- Scuba weight chart, for the buoyancy swing in lead terms
- Gas planning calculator, rock bottom and turn pressure
Frequently asked questions
How much air is in an aluminium 80?
An aluminium 80 holds 77.4 cubic feet of air at its 3,000 psi service pressure, not the 80 the name suggests. That works out to about 2.58 cubic feet per 100 psi. Ending a dive on a 500 psi reserve leaves roughly 64.5 cubic feet usable, which at a moderate breathing rate of 0.6 cubic feet per minute is about 38 minutes at 60 feet.
Why does an aluminium tank float when it is empty?
Because the gas inside it has real weight. Air weighs about 0.0807 lb per cubic foot, so the 77.4 cubic feet in a full aluminium 80 weighs roughly 6.2 lb, and the cylinder gets that much more buoyant as you breathe it. An aluminium 80 runs about 4 lb positive at 500 psi. That is why a weight check is done at the end of a dive, not the start.
Are steel tanks better than aluminium?
They are different rather than better. Steel cylinders carry more gas for their physical size and stay negatively buoyant throughout the dive, so you carry less lead on the belt. Aluminium is cheaper, more corrosion tolerant in salt water, and is what almost every rental fleet and travel destination stocks. For a first owned cylinder, aluminium is usually the practical answer.
How often does a scuba tank need testing?
In the United States a cylinder needs a visual inspection annually and a hydrostatic test every 5 years. Fill stations enforce both, and a cylinder that is out of date will simply be refused. Older aluminium cylinders made from 6351 alloy also need an eddy current test of the neck threads alongside the annual visual, which most inspection shops carry out as routine.
What does the buoyancy figure on a tank spec sheet mean?
It normally means buoyancy in salt water, quoted both full and empty, with a specific valve fitted and no boot. All three of those variables move the number, which is why published figures differ between sources for what looks like the same cylinder. Treat any quoted buoyancy as approximate, and let your own weight check at the end of a dive be the figure you actually use.
Should I buy my own tanks or keep renting?
Rent until your local diving is frequent enough that fill and rental fees exceed the cost of ownership plus annual inspection and hydro testing. Cylinders are heavy, awkward to store, cannot be flown, and need money spent on them every year whether or not you dive. Most divers correctly buy a computer, a mask and exposure protection years before they buy a cylinder.
How we choose: we compare published manufacturer specifications, published training agency standards, and verified owner reviews across retailers. We do not test gear in person, and every depth rating, cylinder capacity and algorithm name quoted here is the manufacturer's published figure unless we say otherwise, so confirm it on the current spec sheet before you buy. Nothing here is dive instruction, and no calculator output on this site is a dive plan. Scuba diving carries a real risk of decompression sickness, oxygen toxicity, barotrauma and drowning. Dive within the limits of your certification, verify every plan with your own computer, and buy the training before the gear that assumes it.
Logging your own SAC rate and gas plans? The Dive Kit & Air Planning Workbook is the paid version of these pages: 8 printable worksheets you fill in with your own numbers, plus the full PDF, $29.