SAC and RMV: Calculate Your Air Consumption

Why Air Consumption Matters

Running out of air is the most preventable accident in diving. Knowing your air consumption rate lets you plan dives precisely — how long you can stay at depth, when to turn around, and what size tank you need for a given dive profile. It also serves as a direct measure of skill progression: as your buoyancy improves and your breathing slows, your air consumption decreases noticeably over time.

Divers who track their SAC and RMV from day one gain a concrete, data-driven view of their improvement. It is one of the most motivating numbers in your dive log.

What Is SAC Rate?

Surface Air Consumption (SAC) rate describes how much gas you breathe per minute, normalized to surface conditions. Because pressure increases with depth — roughly 1 additional atmosphere every 10 metres of seawater — you breathe physically denser gas the deeper you go. At 20 m you are breathing at 3 ATA, so each breath contains three times as many molecules as at the surface.

SAC removes that depth variable so you can compare consumption across dives at different depths. It is expressed in bar/min (metric) or psi/min (imperial) and is tied to the specific cylinder you used, since a bar from an 11 L tank represents more actual gas than a bar from a 7 L tank.

What Is RMV?

Respiratory Minute Volume (RMV) is the actual volume of gas you breathe per minute at the surface, measured in litres/min (metric) or cubic feet/min (imperial). Unlike SAC, RMV is tank-independent — it does not depend on the cylinder's volume or working pressure, only on how much gas your body actually consumed.

This makes RMV the more versatile figure for cross-cylinder planning. If you know your RMV is 14 L/min, you can calculate your gas supply on any tank — an AL80, a 12 L steel, or a set of doubles — without needing to convert between SAC rates for different cylinder sizes.

💡 Key Distinction

SAC is useful for quick planning when you always dive the same tank. RMV is better when you dive a variety of cylinders or want a single number that describes your breathing rate independent of equipment.

Step-by-Step Calculation

Let's work through a real dive example. You started with 200 bar, ended on 80 bar, used an AL80 cylinder (11.1 L internal volume), dove to an average depth of 18 m in saltwater for 45 minutes.

Worked Example — Metric

Step 1. Calculate gas used (bar consumed):

Step 2. Convert to free litres (actual volume at surface pressure):

Step 3. Calculate the pressure in ATA at average depth (saltwater: +1 ATA per 10 m):

Step 4. Divide by time and depth factor to get RMV and SAC:

Gas used    = 200 − 80 = 120 bar
Free litres = 120 × 11.1 = 1,332 L
ATA        = (18 ÷ 10) + 1 = 2.8 ATA

RMV = (1,332 ÷ 45) ÷ 2.8 = 10.6 L/min
SAC = (120 ÷ 45) ÷ 2.8  = 0.95 bar/min
          

Formulas

RMV = (Gas Used × Tank Volume) ÷ Time ÷ ATA where ATA = (Depth ÷ 10) + 1 [saltwater, metres]

SAC = Gas Used ÷ Time ÷ ATA [result in bar/min for the tank used]

Gas Used = start pressure − end pressure (bar) Tank Volume = internal volume in litres (e.g. 11.1 L for AL80) Time = bottom time in minutes ATA = absolute pressure at average depth

📌 Freshwater Note

In freshwater, use (Depth ÷ 10.3) + 1 for ATA, since freshwater is less dense than saltwater. For recreational diving the difference is small, but it matters for precision planning.

What SAC / RMV Rating Should I Have?

RMV benchmarks give you an honest picture of where you sit relative to other divers. There is no shame in a high number — it simply tells you where to focus your training.

RMV	Rating	What It Means
< 12 L/min	Excellent	Very efficient breather — experienced or highly fit
12 – 16 L/min	Good	Typical experienced recreational diver
16 – 20 L/min	Average	Normal for regular recreational diving
20 – 25 L/min	Above Average	Room to improve — focus on buoyancy
> 25 L/min	High	Work on breathing technique and buoyancy control

New divers commonly breathe 20–30 L/min, often because of anxiety, unfamiliar gear, and inefficient finning. With experience and refined buoyancy most recreational divers settle into 12–16 L/min. At the extreme end, world-class rebreather divers may reach 6–8 L/min through years of deliberate breathwork practice.

Tips to Improve Your Air Consumption

Perfect your buoyancy. Fighting to stay at depth is exhausting and gas-hungry. A neutrally buoyant diver barely moves their fins to hold position. Buoyancy is the single biggest lever on air consumption.

Breathe slowly and fully. Avoid shallow, rapid breathing. Slow, complete breaths allow full gas exchange and prevent CO₂ buildup, which is itself a driver of breathlessness underwater.

Streamline your gear. Dangling gauges, loose straps, and poorly routed hoses create drag. Drag makes you kick harder. Kicking harder makes you breathe harder. Clip it all down.

Stay calm and relaxed. Anxiety and stress are among the fastest ways to burn through a tank. Dive well within your comfort level and build exposure gradually.

Improve your cardiovascular fitness. General aerobic fitness lowers your resting breathing rate and reduces the effort required to move through the water, particularly in current.

Track it every dive. Log your SAC or RMV after every dive. Watching the number trend downward over months is one of the most satisfying measures of real progress as a diver.

Using SAC / RMV for Gas Planning

Once you know your RMV, you can estimate how long any cylinder will last at any depth. The formula is:

Tank Duration Formula

Duration = (Usable Gas in Litres) ÷ (RMV × ATA) Usable Gas = (Full pressure − Reserve pressure) × Tank Volume

Usable Gas = gas above your reserve, in litres RMV = your respiratory minute volume (L/min) ATA = absolute pressure at planned depth

Planning Example

RMV of 14 L/min, diving an AL80 (11.1 L, full at 207 bar) to 20 m in saltwater, keeping a 50 bar reserve.

Usable Gas = (207 − 50) × 11.1 = 1,742 L ATA = (20 ÷ 10) + 1 = 3.0 Duration = 1,742 ÷ (14 × 3.0) = ≈ 41 minutes

This is your absolute maximum bottom time on gas alone. In practice, always apply a conservative turn-around rule (e.g. the Rule of Thirds for penetration diving) and account for the fact that your RMV rises with exertion and stress. The number above is a planning ceiling, not a target.

Our SAC Rate Calculator and Tank Duration Calculator automate this arithmetic so you can run scenarios for any depth, tank, and RMV instantly.

Frequently Asked Questions

What's a good SAC rate for a beginner?

Most newly certified divers breathe in the range of 20–30 L/min RMV. This is entirely normal — you are managing unfamiliar equipment, adjusting buoyancy actively, and often diving with some background anxiety. A more useful goal than hitting a specific number early on is to track the trend: if your RMV is lower on dive 20 than it was on dive 5, you are moving in the right direction.

Does SAC change with depth?

SAC rate (in bar/min) does not change with depth by definition — that is exactly what the normalization calculation is designed to remove. However, your actual gas consumption in litres per minute increases proportionally with depth because you are breathing denser gas. RMV, which measures true volumetric consumption at the surface equivalent, also should not change with depth in theory, though exertion, thermal stress, and narcosis can raise your breathing rate on deeper dives.

Should I use SAC or RMV for planning?

Use RMV for planning across different tanks, since it is tank-independent. If you always dive the same cylinder, SAC is perfectly adequate and slightly simpler to apply. When planning with doubles, sidemount, or staged tanks, RMV is the cleaner choice because you can apply a single number to any gas supply configuration without recalculating SAC for each cylinder size.

How many dives do I need to get an accurate SAC/RMV?

A single dive gives you a data point, but conditions vary — water temperature, current, dive site anxiety, and exertion level all affect the result. Average your SAC or RMV across at least 5–10 dives in similar conditions to get a reliable baseline. Many divers keep a rolling average in their log and update it after each dive.

SAC and RMV Explained