Scuba SAC Rate: How to Calculate It, and What's Good

SAC rate (Surface Air Consumption) is how much gas you’d use per minute if you were breathing at the surface — calculated from your pressure used, bottom time, and average depth. It’s the single clearest number for whether your diving is becoming more efficient, because unlike a raw “tank lasted 40 minutes” figure, it accounts for depth and lets you compare dives that were nothing alike.

The formula, worked through

Step 1 — find your ATA (atmospheres absolute) at average depth. Metric: ATA = (average depth in m ÷ 10) + 1 Imperial: ATA = (average depth in ft ÷ 33) + 1

Step 2 — divide pressure used by bottom time and ATA. Metric: SAC = pressure used (bar) ÷ (bottom time × ATA) → bar/min Imperial: SAC = pressure used (psi) ÷ (bottom time × ATA) → psi/min

Step 3 — convert to RMV if you want a tank-independent number. Metric: RMV = SAC × cylinder volume (L) → L/min

Example. A 40-minute dive averaging 20m, breathing a 200-to-60-bar cylinder of 11.1 liters: ATA = (20÷10)+1 = 3 SAC = (200−60) ÷ (40×3) = 140 ÷ 120 = 1.17 bar/min RMV = 1.17 × 11.1 ≈ 13 L/min

What counts as good

A typical recreational SAC rate lands around 15–20 L/min. Divers with strong buoyancy control and relaxed breathing often see it drop under 15 L/min; new divers commonly run above 25 L/min, mostly from overweighting and the extra effort of maintaining trim (Blue Grotto Dive Resort, RMV vs. SAC). The number moves with conditions too — expect it higher in cold water, in current, or on a stressful first dive at a new site, and don’t read too much into any single dive.

Why RMV, not just SAC, if you dive different tanks

SAC is tied to whatever cylinder you happened to be breathing — a bar per minute means something different on a 10L steel tank versus a 15L. RMV strips the tank size out and gives you a liters-per-minute figure that’s comparable across every dive you log, regardless of what you were breathing from. If you only ever use one cylinder size, the distinction barely matters. If you rent gear that varies by dive shop, it matters a lot.

What actually moves the number

Buoyancy control and trim account for most of the swing between a new diver’s SAC and an experienced one’s — a diver constantly finning to correct depth burns more air than one holding neutral without effort. Overweighting compounds it, since a diver working to stay off the bottom breathes harder than one perfectly balanced. Cold, current, anxiety and physical exertion all raise it temporarily; none of them are things to fix, just things to expect on a given dive.

FAQ

Do I need my dive computer to calculate SAC, or can I do it from memory? You need your average depth and pressure used, both of which any dive computer logs automatically — write them down along with bottom time and you have everything the formula needs.

Why does my SAC rate look different on every dive? Depth, exertion, water temperature and stress all shift it — track the trend across many dives rather than judging any single one.

Is a lower SAC rate always better? Generally yes as a sign of relaxed, efficient breathing, but don’t hold your breath or breathe unnaturally shallow to chase a number — that’s a real safety risk, not an efficiency gain.

What’s the difference between SAC and RMV again? SAC is tied to your specific cylinder; RMV is the same calculation converted to a liters-per-minute figure independent of tank size, so it’s comparable across different cylinders.

Skip the manual math: the free Dive Log & Gas Tracker calculates SAC and RMV for every dive automatically and tracks the trend across your logged dives.