SAC / RMV Calculator

Understanding SAC and RMV: The Diver’s Guide to Gas Planning

SAC vs RMV — two views of the same number

Surface Air Consumption (SAC) and Respiratory Minute Volume (RMV) both describe the same physical reality — how much gas you breathe per minute — but they express it in different units. SAC measures consumption in pressure units per minute (bar/min or psi/min) and is tied to a specific tank. RMV measures it in volume units per minute (litres/min or cubic feet/min) and is universal — independent of what cylinder you are using.

Think of it this way: SAC tells you how fast the needle on your pressure gauge drops at the surface. RMV tells you the actual volume of gas passing through your lungs at the surface. Both are normalised to one atmosphere of ambient pressure, which is why they are called “surface” rates — at depth, you actually breathe more gas per minute because the gas is compressed.

The conversion between the two is simple. In metric: RMV (L/min) = SAC (bar/min) × tank water volume (L). A 12-litre tank with a SAC of 1.0 bar/min gives an RMV of 12 L/min. The same diver using a 15-litre tank would show a SAC of only 0.8 bar/min — consuming pressure more slowly — but the RMV remains 12 L/min because their lungs are doing exactly the same work.

Why gas planning matters

Running low on gas underwater is one of the most preventable emergencies in diving. Every year, incident reports from DAN and other safety organisations cite out-of-gas situations as a contributing factor in accidents ranging from panicked ascents to fatalities. A diver who knows their RMV can predict — before the dive — whether a given cylinder carries enough gas for the planned profile plus a safety reserve.

Gas planning also enables better dive decisions on the surface. Should you take a 10-litre or 12-litre tank? Is one fill enough for a 40-minute dive at 25 m, or do you need to start at 230 bar instead of 200? Can you do a second dive on the same fill? These questions all reduce to arithmetic once you know your consumption rate.

The formula, step by step

The SAC formula accounts for the fact that at depth you consume more gas per minute because the ambient pressure compresses the gas to a higher density. To get a surface-normalised rate, you divide the total pressure consumed by both the time and the ambient pressure at your average depth:

SAC = (P_start − P_end) ÷ (time × P_amb)

Where P_amb is the ambient pressure at your average depth in bar: depth_m ÷ 10 + 1. This is the standard hydrostatic approximation used across all dive-planning literature — strictly, one atmosphere is 1.013 bar, but the ÷10 simplification is universal in recreational and technical dive planning and the difference is well below the noise of real-world conditions. Source: PADI / DSAT, Diving Knowledge Theory Folder (2009), Ch. 1.IV, p. 13.

The result is in bar per minute (or psi per minute in imperial). Multiply by your tank’s water volume in litres to convert to RMV in litres per minute. In imperial, the equivalent is: RMV (ft³/min) = SAC (psi/min) × (tank rated ft³ ÷ rated psi).

From log to plan — using RMV to project gas needs

Once you have your RMV, projecting gas for a planned dive is the formula in reverse. The total gas needed (in litres at surface pressure) for a given profile is:

Gas_needed (L) = RMV × time × P_amb

The ambient pressure is calculated at the planned average depth, not the maximum depth. For a square-profile dive where you spend most of the time at one depth, average and max are nearly the same. For a multilevel profile, use the average depth your computer would report — this is always shallower than max depth, so the gas projection is conservative in the right direction.

Add your reserve gas to the consumption figure to find the total gas you need in the cylinder at the start. A common recreational convention is a 50 bar (700 psi) reserve, though this is a convention, not a physics-based calculation. Technical and cave divers use rule-based reserves (rock-bottom, rule of thirds) that account for depth-dependent consumption during an emergency ascent.

Factors that affect your consumption rate

Your RMV is not a fixed number — it varies between dives and even within a single dive. Understanding what moves the needle helps you plan more accurately and identifies areas for improvement.

• Depth. The SAC formula already normalises for ambient pressure, so in theory your RMV should be constant at any depth. In practice, many divers breathe harder at depth due to increased gas density, psychological stress, and the work of breathing through a regulator at high ambient pressure. If you consistently measure a higher RMV on deeper dives, factor that into your planning.
• Workload. Finning against a current, carrying heavy gear, performing tasks, or swimming long distances all increase oxygen demand and therefore gas consumption. A resting diver might use 10–12 L/min; the same diver fighting a current could spike to 25–30 L/min or more.
• Thermal stress. Cold water increases metabolic rate as the body works to maintain core temperature. Divers in cold-water environments routinely report higher consumption than in tropical conditions, even at the same depth and workload.
• Fitness and experience. Cardiovascular fitness, efficient buoyancy control, relaxed breathing patterns, and streamlined trim all reduce gas consumption. New divers and infrequent divers typically consume more gas than experienced divers, sometimes substantially more.
• Stress and anxiety. Psychological stress — whether from task loading, poor visibility, unfamiliar environments, or equipment issues — is one of the fastest ways to increase your consumption rate significantly. This is why gas planning should always include a meaningful reserve for the unexpected.
• Equipment. A poorly maintained or unbalanced regulator can increase the work of breathing. Overweighting forces more effort to maintain trim. An ill-fitting exposure suit can restrict chest expansion.

Improving your consumption rate

Gas efficiency is a skill, not a talent. Most divers can substantially improve their SAC rate with deliberate practice.

• Buoyancy and trim. The single largest factor. A diver who constantly adjusts their BCD, sculls with their hands, or fins to maintain depth is doing unnecessary work. Neutral buoyancy achieved through proper weighting and breath control reduces both effort and gas consumption.
• Breathing technique. Slow, deep breaths are more efficient than rapid, shallow ones. Exhale fully; dead-space gas (the air that stays in your airways without participating in gas exchange) is a larger fraction of each breath when breaths are small and fast.
• Streamlining. Dangling gauges, loose hoses, and poor body position increase drag. Tucking gear close to the body and maintaining a horizontal trim reduces the finning effort needed to move through the water.
• Relaxation. Experience builds confidence, and confidence lowers breathing rate. Logging more dives is the most reliable way to reduce consumption over time.

Track your SAC or RMV across multiple dives to establish your personal baseline. Use this tool after every logged dive to build a running average. Over time, you should see the number trend downward as your skills improve — and you will notice when it spikes, which usually signals a dive where something made you work harder than usual.

Gas planning for sidemount and technical diving

The Mode 2 panel of this calculator supports multi-cylinder gas planning. For sidemount diving, the convention is balanced consumption — drawing equally from both (or all) cylinders so that if one fails, the remaining gas is maximised. The tool divides the total gas requirement evenly across the number of cylinders and shows the per-tank pressure draw.

Technical diving introduces additional gas-planning considerations beyond bottom-gas consumption: decompression gas, stage bottles, bailout gas for CCR failures, and minimum-gas calculations for emergency ascents. This tool covers bottom-gas planning only. For deco planning, use a verified desktop application such as MultiDeco or Subsurface, or a deco-capable dive computer.

Common planning mistakes

• Using max depth instead of average depth. SAC calculated with max depth will understate your true consumption rate, because you were not at maximum depth for the entire dive. Always use the average depth your computer reports.
• Ignoring descent and ascent gas. The gas consumed during descent and ascent is part of your total consumption and is captured in the SAC formula when you use average depth and total bottom time. When projecting a plan, remember that descent and ascent phases consume gas too — a square-profile projection is a reasonable approximation for recreational diving.
• Assuming your SAC is constant. Your consumption varies by dive. Use a planning RMV that reflects realistic conditions, not your best-ever number from a calm, warm, shallow dive. Many tech instructors recommend adding 20–30% to your measured baseline for planning purposes.
• Forgetting the reserve is not for the dive. The reserve (50 bar, or whatever convention you use) is emergency gas for situations where things go wrong — not a buffer for extending the dive. Plan your bottom time so you surface with the reserve intact.