How to Calculate Your SAC Rate and Lower Your Air Consumption

How do you calculate SAC rate? A worked example

SAC (surface air consumption) is the gas you breathe per minute, scaled back to surface pressure so that dives at different depths can be compared. You need five numbers from a dive you have already done: start pressure, end pressure, tank size, dive time and average depth. Our SAC calculator uses exactly the steps below.

Say you started at 200 bar and surfaced with 70 bar in a 12 L tank. The dive lasted 48 minutes, and your computer shows 18 m (60 ft) maximum and 12.4 m (41 ft) average depth.

• Gas used: (200 - 70) × 12 = 1,560 L.
• Average absolute pressure: 12.4 ÷ 10 + 1 = 2.24 bar.
• Gas per minute at that pressure: 1,560 ÷ 48 = 32.5 L/min.
• SAC: 32.5 ÷ 2.24 = 14.51, rounded to 14.5 L/min.

Why average depth and not maximum depth?

Because the gas was breathed across the whole profile, and the average describes that profile. Put the 18 m maximum into the last step and the divisor becomes 2.8, so the SAC drops to 11.6 L/min. That is 20% below what you actually breathed. The dive spent most of its time shallower than 18 m, so the maximum makes you look more economical than you are, and any plan built on that number comes up short.

The depth conversion treats 10 m of seawater as 1 bar. The US Navy Diving Manual uses the same definition for metric depths, and DAN Europe's RMV method adds 1 bar for the surface in the same way. Take the average from the computer's dive log rather than estimating it from memory.

What is a SAC number useful for?

Mainly gas planning. SAC times absolute pressure gives litres per minute at depth, and times minutes gives the gas a dive needs. With the 14.5 L/min above, a 40-minute dive at an 18 m average needs 14.5 × 40 × 2.8 = 1,624 L. In a 12 L tank that is 135.3 bar, which our gas planner rounds up to 140 bar.

DAN Europe, which calls the same litres-per-minute figure RMV, recommends rounding up for planning (its example treats 13.4 L/min as 14) and basing a team plan on the highest rate in the group. Rounded up to 15 L/min, the same dive needs 1,680 L, still 140 bar.

SAC also feeds the emergency side of the plan. The planner's rock bottom figure assumes two divers sharing one tank at a stressed SAC, and it suggests double your normal rate as the starting point. Last, a SAC worked out after every dive tells you whether a change to weighting, trim or kit made any difference.

What is a normal SAC rate?

There is no single normal figure, because breathing follows workload. The US Navy Diving Manual puts respiratory minute volume at about 6-10 L/min at complete rest and over 100 L/min during severe work. The table below fills in the middle from its activity chart and from a 2019 open-water study of 18 divers.

The Navy figures are lung volumes at body temperature (BTPS), a slightly different basis from a gauge-based SAC. The study figures are litres at ambient pressure, which is what a SAC measures. Read both as scale rather than targets. The pattern is what matters: doubling swim speed, 0.5 knot against 1 knot, more than doubles the breathing rate. For its own scuba planning the Navy assumes 1.4 cu ft/min (about 40 L/min) and tells supervisors to assume more when tasks or conditions call for it.

What raises your SAC: workload, cold and stress

Work is the biggest lever. The Navy manual says breathing volume changes in proportion to oxygen consumption, so anything that makes your muscles work harder shows on the gauge. Its list of factors that influence a diver's work rate includes water temperature, thickness of thermal protection, current and visibility, the task and experience with it, fitness, and how recent that experience is. DAN notes that lobster hunting or spearfishing can use gas 5-10 times faster than usual.

Cold adds work you can't see. StatPearls describes shivering pushing metabolic heat production up to several times the resting rate, and more oxygen consumed means more gas breathed. DAN's incident review describes a diver on a cold night dive in an unfamiliar drysuit whose poor buoyancy control and stress drained the tank faster than usual.

Stress shows up directly. In the 2019 study, fast descents at 23 m/min averaged 28 L/min, 82% more than the same divers at the 11 m stage, with heart rate 23% higher. The authors concluded that physiology alone could not explain the difference and named mental stress as a likely main contributor. Slow descents at 2.4 m/min cost 17 L/min. For dives deeper than 20 m they recommend descending at about 10 m/min.

How trim and overweighting push SAC up

Through drag. A diver swimming at an angle pushes more water out of the way, and moving water costs oxygen. In a 1996 study of scuba swimming energetics, gear placement that changed the diver's attitude in the water raised the energy cost of swimming by 30% at slow speeds. Adding extra tanks or a drysuit raised it by 25%. Fins mattered as well: oxygen cost ran 25-50 L per kilometre depending on the fin, and a large flexible fin was the cheapest in that study.

Overweighting is a common way to end up tilted. DAN lays out the chain: extra lead means more air in the BC to stay neutral, that air can put the diver in a more upright profile, and the upright profile increases drag, effort and air use. DAN also points out that many overweighted divers don't realise they are. Going light is no fix either, because underweighted divers can tire themselves out fighting to stay down.

Why two divers on the same dive get different SAC rates

SAC removes depth from the comparison, not the diver. The 1996 energetics study measured the oxygen cost of swimming at about 30 L per kilometre for women and 50 for men, and experienced male divers used less oxygen than beginners. DAN Europe adds that RMV tends to fall with experience and rise when a diver hasn't been in the water for a while.

Some of the gas in a logbook SAC was never breathed at depth. Every bar that leaves the tank counts, including BC and drysuit inflation and any breathing from the regulator at the surface before descending. Shearwater's Perdix AI discards the first 30 seconds of each dive from its SAC for exactly this reason, to leave out the gas typically used inflating a BC, wing or drysuit.

How to lower your SAC rate

Cut effort and drag first. Breathing tricks come last, and one of them is harmful.

• Swim slower. The Navy chart above shows breathing climbing faster than speed, so covering less ground is the cheapest saving available.
• Get the weight right for the day. DAN notes that aluminium tanks, thicker neoprene, drysuits and salt water all need more lead, so the right amount changes between trips. Do a buoyancy check with the actual suit, tank and water.
• Level out. DAN says a level profile makes you more hydrodynamic. Spread weights evenly from side to side and place them so you hang horizontal.
• Stay warm. A drysuit added 25% to swimming cost in the 1996 study, while shivering can multiply resting heat production. Dress for the water temperature.
• Descend under control instead of dropping fast, especially on deeper dives.
• Dive regularly, and expect the first dive after a break to cost more gas.
• Don't skip-breathe. Holding each breath to stretch the gas lowers ventilation relative to effort, and the Navy lists skip breathing as a cause of carbon dioxide buildup (hypercapnia).

How to track SAC across dives

Work it out after every dive from the same logbook fields, and compare dives of the same kind. A drift dive and a slow photo dive are different workloads, so a single average across both hides more than it shows.

• Record start and end pressure, tank size, dive time and average depth on every dive. Our guide to logging a dive covers the rest of the entry.
• Write the SAC next to the dive, with a note on anything unusual: water temperature, current, a new suit, a camera, a task.
• Plan on a recent average rounded up, and recalculate after a long break, as DAN Europe suggests.
• If your computer is air-integrated, check the unit. Shearwater's Perdix AI shows SAC in bar (or psi) per minute, which does not transfer between tank sizes. Multiply by the tank's litres to get L/min: 1.2 bar/min on a 12 L tank is 14.4 L/min.