Air Consumption and Gas Planning for New Divers

Your air runs out faster the deeper you go because every 10 metres of seawater adds about 1 bar of pressure, and your regulator hands you each breath at that pressure. Gas planning is simply knowing how fast you personally use air, agreeing the gauge reading at which you turn around, and getting back to the boat or shore with a reserve, conventionally at least 50 bar.

This is general information with worked examples in bar and litres; on the day, your instructor or dive briefing sets the plan.

Why Air Runs Out Faster as You Go Deeper

At the surface you are under about 1 bar of pressure from the atmosphere. Every 10 m of seawater adds roughly another bar, so the total (called absolute pressure) is about 2 bar at 10 m, 3 bar at 20 m and 4 bar at 30 m.

Your regulator delivers air at whatever pressure surrounds you. That lets your lungs fill normally at depth, but each breath at 10 m holds about twice as much air as the same breath at the surface; DAN describes air at about 30 m as four times as dense as at sea level. The US Navy Diving Manual puts numbers on it: a cylinder that would last a gently swimming diver about 100 minutes at the surface lasts about 50 minutes at 10 m, and only a third as long at 20 m.

| Depth | Absolute pressure | Air in each breath (vs surface) | How long a tank lasts (vs surface) | |---|---|---|---| | 0 m | 1 bar | 1x | 100% | | 10 m | 2 bar | 2x | 50% | | 20 m | 3 bar | 3x | 33% | | 30 m | 4 bar | 4x | 25% |

The physics behind it, Boyle's law, is part of your course's theory module.

Surface Air Consumption: SAC and RMV in Plain English

Divers compare breathing rates by converting them back to the surface, in two ways:

- SAC (surface air consumption): how many bar a minute you would use at the surface. It is easy to read off your gauge, but it only applies to a cylinder of the same size, because 1 bar in a 15 litre cylinder is more air than 1 bar in a 10.5 litre one. - RMV (respiratory minute volume): how many litres of air a minute you breathe at the surface. It works with any cylinder, so it is the more useful number to know.

What is normal? The US Navy Diving Manual gives about 6 to 10 litres a minute at rest and about 20 for a diver swimming gently. In a study of more than 1,000 organised recreational dives in Western Australia, average consumption was about 0.22 to 0.23 litres a minute per kilogram of body weight, roughly 18 litres a minute for an 80 kg diver. It improves with experience: PADI's first tip is simply to dive more.

How to Measure Your Own

DAN's Alert Diver magazine describes a simple timed swim. On an easy dive, with your buddy in on the plan, settle at a steady depth over a clear reference such as a flat sandy bottom, note your depth and gauge reading, swim at your normal pace for 10 minutes while holding that depth, then note the gauge again. Afterwards:

- Bar per minute at depth = bar used ÷ minutes - SAC = bar per minute ÷ absolute pressure (depth in metres ÷ 10, plus 1) - RMV = SAC × cylinder size in litres

Worked example. At 10 m with a 12 litre cylinder, the gauge drops from 170 to 140 bar in 10 minutes. That is 3 bar a minute at depth. The absolute pressure at 10 m is 2 bar, so your SAC is 3 ÷ 2 = 1.5 bar a minute, and your RMV is 1.5 × 12 = 18 litres a minute.

Now plan a dive to 18 m, where the absolute pressure is 2.8 bar: 18 × 2.8 = about 50 litres a minute, or about 4.2 bar a minute from that same cylinder.

DAN suggests repeating the swim at harder and easier paces, since a current pushes the number up and a lazy drift brings it down. Any figure is an estimate, never a replacement for your gauge.

An air-integrated computer, which reads tank pressure from a transmitter on your regulator, can do the maths for you. Shearwater's records your SAC and shows "gas time remaining", the minutes you could stay at this depth and still surface with your reserve. Our dive computer guide covers when that is worth paying for.

Australian Cylinders: What You Will Breathe From

Australian cylinders are described by internal volume in litres and maximum fill pressure in bar. Retailer The Scuba Doctor says steel dominates in southern waters (it is tough and sinks, offsetting a thick wetsuit) and cheaper aluminium is more common up north. Its most common sizes:

- 12.2 L steel, 232 bar: the size most divers use. - 10.5 L steel, 232 bar: lighter, popular with smaller divers and low breathers. - 15 L steel, 232 bar: for heavy breathers, but bigger and heavier. - 11.1 L aluminium, 207 bar: the warm-water "aluminium 80".

Hire fleets reflect this (checked October 2026). Adelaide's The Dive Shack hires 12 L aluminium and 10, 12 and 15 L steel tanks with yoke or DIN valves, and the Adelaide University Scuba Club hires 12 L Faber steel. Frog Dive in Sydney lists steel and aluminium cylinders in 10 L and 15 L sizes, and Infinity Dive in Chatswood sells 12.2 L and 15 L Faber steel rated to 232 bar. The rating is a maximum: Infinity lists its hire tank as filled to at least 210 bar, so check your gauge before you get in.

Here is how much the cylinder matters for the same diver (RMV 18 litres a minute, about 50 at 18 m), starting at 200 bar and keeping 50 bar in reserve:

| Cylinder | Usable air (200 down to 50 bar) | Rough minutes at 18 m | |---|---|---| | 10.5 L steel | 1,575 L | 31 | | 12.2 L steel | 1,830 L | 36 | | 15 L steel | 2,250 L | 45 |

These are rough: they ignore the air used going down and up, and your computer's no-decompression limit may end the dive first (see what the NDL means). Note too that a 50 bar reserve is 525 litres in a 10.5 L cylinder but 750 litres in a 15 L one.

Planning the Dive: Reserve and Turn Pressure

The Reserve

Fifty bar is the reserve you will hear in most briefings. A Western Australian study in Diving and Hyperbaric Medicine notes that dive leaders there commonly set it as the minimum, partly because many gauges mark the bottom 50 bar in red, and PADI's tips describe hovering at your safety stop with about 50 bar left. DAN adds that some gauges do not read zero when the tank is empty, so surface with at least 500 psi (about 35 bar) showing.

The reserve is not spare dive time. It covers what goes wrong: a buddy who runs low, a delay on the ascent line, a long surface swim. DAN Europe's Alert Diver calls it a shared resource, so the dive ends when the first diver reaches it, and argues that 50 bar is a minimum, not a magic number. Planners call the air needed to get two divers sharing one supply from the deepest point to the surface, safety stop included, "rock bottom" or "minimum gas". In the magazine's example, two divers sharing from 30 m needed about 550 litres, roughly 50 bar in a standard aluminium cylinder, before any safety margin; the author suggested 80 to 100 bar for that dive.

The Turn Pressure

Your turn pressure is the gauge reading at which you head back to the exit or ascent line, agreed with your buddy before you descend. DAN's out-of-air safety tips suggest turning back once you have used half of your available air.

Worked example. You start with 200 bar and plan a 50 bar reserve, leaving 150 bar to use. Half of that is 75 bar, so you turn at 125 bar and still arrive back with your reserve.

If a current carried you out, the swim home will cost more, so turn earlier. In a buddy pair, the diver who breathes faster sets the turn.

In the Western Australian study, 183 of 1,032 logged dives, about one in six, ended below 50 bar, more often on deeper dives, with heavier breathing, and among younger, male or less experienced divers or those who had not dived for a while. The paper also cites an analysis of 859 reported Australian diving incidents in which about one in five involved an out-of-gas problem. The researchers' advice: check your gas often, more so the deeper you go.

The Rule of Thirds: Where It Comes From and When to Use It

The rule of thirds splits your starting air three ways: a third to go out, a third to come back and a third in reserve. It comes from cave diving, where you cannot swim straight up and must return the way you came. It is usually credited to American cave pioneer Sheck Exley, who set it down in his late-1970s booklet Basic Cave Diving: A Blueprint for Survival, and it remains a standard starting point in caves, wreck interiors and under ice.

Recreational divers borrow it. DAN's Alert Diver recommends thirds whenever you must get back to a fixed ascent point, such as a boat's anchor line, and PADI notes it is used in many types of diving. From 200 bar, thirds means turning at about 133 bar and arriving back with about 67.

Two cautions. Thirds assumes you breathe as calmly on the way home as on the way out; cave instructor Stratis Kas has argued in InDepth that stress breaks that assumption, and that Exley meant the rule as a minimum. And knowing the rule does not qualify you to go inside a wreck or cave: that takes specific training.

How to Use Less Air

Most of the gains come from moving less and moving efficiently:

- Get your buoyancy right. Going up and down ("yo-yo" diving) drains your tank and, DAN warns, raises the risk of lung injury and decompression sickness. Our guide to perfect buoyancy covers the skills. - Carry the right weight. Too much lead drags your legs down and needs extra air in your BCD to hold you level; DAN notes that fine-tuning your weighting leaves more air to breathe. See how many weights you need. - Trim and streamline. Water is about 800 times denser than air, so a horizontal body, clipped-in gauges and no dangling hoses all cut the effort (DAN). - Breathe slowly and deeply, and relax. Short, shallow breaths are the worst habit (PADI). Never "skip breathe", holding each breath to stretch your air: DAN lists it among the causes of carbon dioxide build-up. - Slow down. Fin gently. DAN says exertion at depth can drain a tank up to twenty times faster. In a current, stay close to the reef and use it for shelter (PADI). - Stay warm, but not overdressed. The US Navy lists water temperature and suit thickness among the things that change a diver's workload, and notes a thicker wetsuit is more tiring and uses more air. Our wetsuit thickness guide helps match the suit to the water. - Fix leaks. A bubbling o-ring or inflator, or a leaky mask you keep clearing, costs air (DAN). - Go a little shallower if you are worried about a short dive, telling your buddy and guide first, and stay close to your buddy, because searching for a lost one is stressful hard work (PADI).

Check Your Gauge, and Say No When It Is Low

None of this works unless you look. Check your pressure regularly, more often on deeper dives, and make "check gauge, check buddy" a habit on your first dives after certification. DAN's advice is never to start a dive on less than a full tank. If the numbers do not add up, that is a good moment to say no to the dive.

Sources

- DAN (Alert Diver): Estimating Your Air Consumption, Mike Ange, 2010 - DAN: The Most Common Causes of Out-of-Air Emergencies - DAN: Safety Tips to Prevent Out of Air Emergencies - DAN (Alert Diver): The Physiology of Compressed-Gas Diving, 2016 - DAN Europe (Alert Diver): Let's Talk About Gas Planning, Tim Blömeke, 2022 - US Navy (NAVSEA): U.S. Navy Diving Manual, Revision 7 (2016), chapters 2, 3 and 7 (public-release copy hosted by Arizona State University) - Diving and Hyperbaric Medicine: Buzzacott et al., Risk factors for running low on gas in recreational divers in Western Australia (2011) - PADI: 14 Tips to Improve Your Air Consumption - PADI: Bar or PSI, Why It Doesn't Matter Which You Use - Shearwater Research: Getting Started With Air Integration - The Scuba Doctor: Buying a Scuba Cylinder (scubadoctor.com.au/buying-a-scuba-cylinder.htm) - The Dive Shack: Scuba and Snorkeling Hire - Adelaide University Scuba Club: Equipment Hire - PADI: Frog Dive Sydney, dive centre listing - Infinity Dive: Gear Rental and Tank Filling - Infinity Dive: Faber Steel 12.2 L 232 Bar Dive Tank - Infinity Dive: Faber Steel Dive Tank Cylinder 15 L, 232 Bar - Wikipedia: Rule of thirds (diving) - InDepth: Was Sheck Wrong? Stratis Kas

Header photo: A scuba diver climbing the stern ladder back onto a Reef Life Survey boat at Bicheno, Tasmania, by Peter Southwood, CC BY-SA 3.0, via Wikimedia Commons.