SAC Rate Calculator: Surface Air Consumption
Surface air consumption is calculated as (pressure used divided by dive time) divided by average ambient pressure in atmospheres. A diver who uses 2,000 psi in 40 minutes at an average depth of 50 feet has a SAC rate of 19.9 psi per minute, which in an aluminium 80 is an RMV of 0.51 cubic feet per minute. Most recreational divers fall between 0.4 and 0.75 cubic feet per minute.
Your surface air consumption rate, almost always shortened to SAC, is how fast you would breathe down a cylinder at the surface, with the effect of depth mathematically removed. It is the one personal number that makes every other gas calculation on this site work, because without it a gas plan is a guess. The version expressed as a volume, your respiratory minute volume or RMV, is the figure you actually want, and the calculator below produces both.
You need three numbers off a single dive to do this: the pressure you started with, the pressure you finished with, and the average depth over the dive time. A modern computer such as the Shearwater Peregrine logs average depth for you, which removes the largest source of error in the whole calculation. If yours does not, a mechanical submersible pressure gauge plus a written note on a wrist slate at the turn point will get you close enough to be useful.
- Used at depth
- 50.0
- Average pressure
- 2.52 ata
- Band
- Typical
How long that cylinder lasts, by depth
| Depth | Ambient pressure | Consumption | Time to reserve |
|---|
What is a SAC rate, in one sentence?
Surface air consumption is the rate at which you would breathe gas out of a cylinder at the surface, in pressure per minute, once the effect of depth has been divided out. It exists because raw consumption is useless for planning: the same diver breathing exactly the same way uses gas four times as fast at 99 feet as at the surface, so a figure measured at one depth tells you nothing about a dive at another until you normalise it.
The normalising step is a single division. Ambient pressure in atmospheres absolute is the depth divided by 33 feet of sea water, plus 1 for the atmosphere already pressing on the surface. At 33 feet that is 2 ata, at 66 feet 3 ata, at 99 feet 4 ata. Divide your measured consumption by that number and you have a figure that is true of you rather than of the dive.
How do you calculate SAC rate by hand?
SAC (psi/min) = (psi used / dive time) / ((average depth in fsw / 33) + 1)
Work the standard example all the way through. A diver starts on 3,000 psi, surfaces with 1,000 psi, so uses 2,000 psi. The dive lasted 40 minutes at an average depth of 50 feet.
- Consumption at depth: 2,000 divided by 40, which is 50 psi per minute.
- Average ambient pressure: 50 divided by 33, plus 1, which is 2.52 ata.
- SAC: 50 divided by 2.52, which is 19.9 psi per minute.
Note what the average depth means here. It is the time-weighted average that your computer logs, not the halfway point between your maximum depth and the surface, and not your maximum depth. A dive that spends 35 minutes at 55 feet and 5 minutes on a stop has an average depth nowhere near its 70 foot maximum. Using maximum depth instead of average depth is the single most common mistake in this calculation and it understates your SAC substantially, which is the dangerous direction to be wrong in.
Why should you convert SAC to RMV before planning anything?
Respiratory minute volume is the same measurement expressed as gas volume rather than cylinder pressure, and it is the version that survives a change of cylinder.
RMV (cu ft/min) = SAC (psi/min) x (cylinder capacity in cu ft / cylinder service pressure in psi)
An aluminium 80 holds 77.4 cubic feet at its 3,000 psi service pressure, which works out at 0.0258 cubic feet per psi. So the 19.9 psi per minute above becomes 0.51 cubic feet per minute. In metric that is about 14.4 litres per minute.
Here is why it matters. Take that same diver, same breathing, and hand them a steel HP117: 117 cubic feet at 3,442 psi, or 0.034 cubic feet per psi. Their RMV is unchanged at 0.51, but their SAC in that cylinder is 0.51 divided by 0.034, which is 15.0 psi per minute rather than 19.9. Nothing about the diver changed. If you had planned a dive using the psi figure from the aluminium cylinder you would have been wrong by a third, and wrong in the direction of thinking you had less gas than you did, which at least is the safe direction. Reverse the cylinders and the error runs the other way.
| Cylinder | Capacity | Service pressure | Cu ft per psi | SAC at RMV 0.51 |
|---|---|---|---|---|
| Aluminium 40 | 40 cu ft | 3,000 psi | 0.0133 | 38.3 psi/min |
| Aluminium 63 | 63.4 cu ft | 3,000 psi | 0.0211 | 24.2 psi/min |
| Aluminium 80 | 77.4 cu ft | 3,000 psi | 0.0258 | 19.8 psi/min |
| Aluminium 100 | 99.5 cu ft | 3,300 psi | 0.0302 | 16.9 psi/min |
| Steel LP85 | 85 cu ft | 2,400 psi | 0.0354 | 14.4 psi/min |
| Steel HP100 | 100 cu ft | 3,442 psi | 0.0291 | 17.5 psi/min |
| Steel HP117 | 117 cu ft | 3,442 psi | 0.0340 | 15.0 psi/min |
What is a normal SAC rate, and what does a high one mean?
Most adult recreational divers land between 0.4 and 0.75 cubic feet per minute, which is roughly 11 to 21 litres per minute. Around 0.4 is what a relaxed, correctly weighted diver achieves in warm water with nothing to do. Around 0.6 is entirely normal for a diver in their first fifty dives. Above 0.8 something specific is usually happening, and it is almost never lung capacity.
The honest ranking of causes, by how much they actually move the number:
- Overweighting. Every extra pound of lead has to be offset by air in the wing, which increases your frontal area and your drag, and the extra air makes depth control harder so you spend the dive making corrections. Fixing weighting is the single largest change most divers can make. Run your numbers through the weighting calculator and then do a real weight check in the water.
- Trim. A diver swimming at 30 degrees to the horizontal is pushing water with their whole torso. Getting flat can cut effort noticeably without changing anything else.
- Speed. Drag rises with the square of speed, so swimming 30 percent slower cuts drag by roughly half. Slowing down is the cheapest gas saving in diving.
- Cold. Being cold raises metabolic rate whether you notice it or not. A diver who is marginal in a 5 mm suit will show it in the gas figures before they feel it. The wetsuit thickness calculator is a reasonable starting point for whether you are underdressed.
- Task loading and anxiety. Real and temporary. A first dive on a new site, or the first dive after a long break, will read high and settle in a few dives.
Deliberate breath control belongs near the bottom of that list and skip breathing does not belong on it at all. Holding your breath to save gas raises carbon dioxide, which raises the urge to breathe, which raises consumption, and it carries a lung overexpansion risk on any ascent. Breathe normally, slowly and completely, and fix the buoyancy instead.
How many dives do you need before your SAC rate is meaningful?
One dive gives you a data point, not a rate. Consumption varies by 20 percent or more between dives depending on temperature, current, workload and how well you slept, so a single figure is a snapshot of a specific day. Log at least five dives across similar conditions and take a median rather than a mean, because one stressful dive will drag an average badly.
Log the conditions with the number, not just the number. Water temperature, exposure suit, whether there was current, and roughly what you were doing all belong in the entry, because a 0.65 in a 7 mm suit fighting current and a 0.65 in warm water doing nothing describe two very different divers. A written dive log or an app export both work; what matters is that the conditions travel with the figure.
When you plan a dive, use the highest realistic figure rather than your best one. Gas planning is one of the few places in diving where optimism has a direct physical cost, and the number that matters for a reserve calculation is the one you produce on a bad day, not the one you produced on a calm summer afternoon with no current.
How do you use a SAC rate to plan gas?
Two steps, in this order, and the order is not optional.
First, calculate your reserve. Rock bottom is the volume of gas two divers need to get from the planned depth to the surface, sharing one supply, including a minute at depth to sort out the problem and a three minute stop at 15 feet. That number is not negotiable and it is not what is left when you feel like turning. The gas planning calculator works it out for the depth and cylinder you are actually diving.
Second, spend what is left. Usable gas is start pressure minus rock bottom. On a straightforward out and back dive you turn when you have used half of it. If the site has a fixed exit point far from the entry, that half becomes a smaller fraction, because you are committing to swim the distance whatever happens.
A worked example at 60 feet in an aluminium 80, at an RMV of 0.51 cubic feet per minute. Ambient pressure is 2.82 ata, so consumption is 1.44 cubic feet per minute, which is 55.7 psi per minute. Rock bottom for two divers at 60 feet works out around 22 cubic feet, roughly 850 psi. Starting at 3,000 psi leaves 2,150 psi of usable gas, so the turn pressure is 3,000 minus half of 2,150, which is about 1,925 psi, and total bottom time is around 38 minutes. Compare that against the no-decompression limit for 60 feet before you get excited, because on air the table limit will often bite first.
Does nitrox change your SAC rate?
No. Gas density at a given depth is very slightly different between air and a nitrox mix, but not enough to matter at recreational depths, and the calculation is identical either way. What nitrox changes is the nitrogen loading, which is what limits your bottom time on many dives. That is a separate calculation entirely: see the equivalent air depth calculator for the nitrogen side and the nitrox MOD calculator for the oxygen ceiling that comes with it.
The practical consequence is worth stating plainly, because it catches people out. Nitrox often extends your no-decompression limit past the point where your gas supply runs out first. A diver on EAN32 at 60 feet may have well over an hour of table time and only 40 minutes of gas. Once that happens, buying a larger cylinder does more for your bottom time than buying a richer mix, and knowing your own RMV is the only way to see which of the two limits is binding.
Related tools and reading
- Gas planning calculator, for rock bottom and turn pressure
- Nitrox MOD calculator, for the oxygen ceiling on a mix
- Weighting calculator, the biggest single lever on consumption
- Best dive computers, which log the average depth this calculation needs
- Beginner scuba setup, the complete first kit
- Dive computer specifications A to Z
Frequently asked questions
How do you calculate your SAC rate?
Divide the pressure you used by the dive time to get consumption at depth, then divide that by the average ambient pressure in atmospheres. A diver who uses 2,000 psi in 40 minutes at an average depth of 50 feet burns 50 psi per minute at depth. Average ambient pressure is 50 divided by 33 plus 1, which is 2.52 atmospheres. So the surface air consumption is 50 divided by 2.52, or 19.9 psi per minute.
What is a good SAC rate for a diver?
Expressed as a volume rather than a pressure, most adult recreational divers sit between 0.4 and 0.75 cubic feet per minute, which is roughly 11 to 21 litres per minute. Under 0.5 is efficient, and around 0.4 is what a relaxed, well trimmed diver in warm water achieves. A number above 0.8 usually points at cold, poor buoyancy control or overweighting rather than at lung size.
What is the difference between SAC and RMV?
SAC is measured in pressure per minute and RMV in volume per minute. The important consequence is that an RMV figure travels between cylinders and a SAC figure does not. A SAC of 20 psi per minute means something completely different in an aluminium 80 than in a steel 117, because the same pressure drop represents a different amount of gas. Always convert to volume before you plan a dive on a rented cylinder.
Why does air last less time at depth?
Because a regulator delivers gas at ambient pressure, so each breath contains more molecules the deeper you go. At 33 feet the ambient pressure is 2 atmospheres and you consume gas twice as fast as at the surface. At 66 feet it is three times, and at 99 feet four times. Your breathing has not changed at all; the density of what you are breathing has.
Does a bigger cylinder lower your SAC rate?
No. A larger cylinder gives you more gas, so the dive lasts longer, but your respiratory minute volume is a property of you rather than of the cylinder. What a bigger cylinder does change is the SAC figure expressed in pressure, because the same volume of gas is a smaller pressure drop in a bigger cylinder. That is the exact reason to record the volume figure rather than the pressure one.
How do I lower my air consumption?
In order of effect: fix your weighting, then your trim, then your finning, then your breathing. Most high consumption is a buoyancy problem in disguise, because an overweighted diver carries extra air in the wing and swims at an angle, which multiplies drag. Slowing down helps more than any breathing technique, since drag rises with the square of speed. Cold water also raises consumption on its own.
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.