SAC Rate Calculator

Why Your SAC Rate Is the Foundation of Dive Planning

Surface Air Consumption (SAC) rate answers a question every diver needs solved before a dive, not during one: how long will this tank actually last? Rather than guessing from a gauge check partway through, SAC rate gives you a personal breathing-rate figure, normalized to sea-level pressure, that you can re-apply to any combination of tank size and planned depth. Once you know it, gas planning stops being a rough guess and becomes arithmetic — multiply your SAC rate by the atmospheres at a planned depth and the minutes you intend to spend there, and you know roughly how much pressure that dive will cost before you're ever in the water.

Step by Step: What to Log During a Dive

Four numbers are all this calculator needs, and all four should come from the dive itself rather than an after-the-fact estimate: starting tank pressure, ending tank pressure, total bottom time, and average depth. The most common logging mistake is substituting maximum depth for average depth — a dive that spends two minutes at 90 feet and the rest of an hour at 20 feet has a very different average than its deepest point, and using max depth will noticeably inflate your apparent consumption. A dive computer's logged average depth is far more reliable than a mental estimate; if yours doesn't report one, treat any manual guess as rough and read the resulting SAC rate with that in mind.

The Formula Explained: Pressure, ATA, and Volume

Start with pressure used (starting minus ending) divided by dive time, which gives your consumption rate in PSI or bar per minute at depth — but "at depth" is the catch, since the deeper you are, the denser the air you're breathing, so identical lung effort drains more actual gas from the tank. Atmospheres absolute (ATA) corrects for that: ATA = depth in feet ÷ 33 + 1 in saltwater (÷10 + 1 for meters), so 60 feet works out to 2.82 ATA — you're consuming gas 2.82 times faster than the same breathing effort would use at the surface. Dividing your at-depth rate by ATA gives the SAC rate proper: a surface-equivalent pressure drop per minute, still expressed in PSI or bar and therefore still tied to your specific cylinder. Converting that into an actual gas volume per minute — cubic feet or liters, sometimes labeled RMV rather than SAC — uses your tank's own rated volume and pressure, which is what makes the number comparable across different cylinders.

A Real Example

Take a diver on an aluminum 80 tank (rated 80 cu ft at 3,000 PSI) who starts a 40-minute dive at 3,000 PSI and surfaces at 1,000 PSI, averaging 60 feet. That's 2,000 PSI used over 40 minutes, or 50 PSI/min at depth. At 60 feet, ATA is 2.82, so the SAC rate is 50 ÷ 2.82 ≈ 17.7 PSI/min at the surface. Running that through the AL80's tank factor (80 ÷ 3,000 = 0.0267) gives an RMV of about 0.47 cubic feet per minute — comfortably inside the typical experienced-diver range of roughly 0.4 to 0.6 cubic feet per minute, with newer or more anxious divers often running noticeably higher.

Edge Cases to Watch For

A very shallow dive (average depth under 15 feet or so) barely raises ATA above 1, so SAC rate and raw at-depth consumption end up nearly identical — the formula still works, it just has less to correct for. Multi-level dives are a bigger caveat: averaging one number across a profile that spent real time at several different depths is a simplification, and a dive computer's logged consumption, which integrates pressure against the full depth trace, will always be more precise than a single average-depth calculation. Finally, don't treat your SAC rate as a fixed personal constant — cold water, current, task loading, and plain stress all raise it, sometimes by 50% or more over a calm, warm reference dive, so it's worth calculating from a handful of dives rather than trusting just one.