What Is the 1/3 Rule in Diving?

The rule of thirds is how divers manage their air so they don't run out in the wrong place. Use a third of your tank going out, a third coming back, and keep a third in reserve. That last third is your safety buffer for anything that goes sideways.

How It Works

Say you start a dive with 200 bar. One third is about 67 bar.

  • First third (200 to 133 bar): Swim out, explore, go deeper. This is your outbound leg.
  • Second third (133 to 67 bar): Turn around and head back to your exit point.
  • Third third (67 to 0 bar): Don't touch this. It's your emergency reserve.

When your gauge hits two-thirds remaining, you turn the dive. Doesn't matter how good the viz is or what you've just spotted. You turn.

Where Did It Come From?

Cave divers. The rule of thirds was developed for overhead environments where you can't just go straight up if something goes wrong. In a cave or a wreck, you have to swim all the way back out the way you came in. Running low on air in an overhead environment can kill you.

The rule spread from cave diving into wreck diving and eventually into general recreational diving. Most training agencies now teach some version of it as part of air management, even for open water divers.

Why a Third in Reserve?

Because things go wrong. Your buddy runs out of air and needs to share yours. A current picks up on the way back and you're working harder than expected. You get disoriented and take longer to find the exit. Silt gets kicked up and you lose visibility.

A third of your tank gives you enough gas to handle one or two of those problems and still get out safely. It's not excessive. In overhead environments, it's the minimum.

WorkSafe Queensland identifies air supply failure and drowning as hazards that apply to diving activities most of the time. The rule of thirds exists specifically to manage that risk.

Does It Apply to Open Water Diving?

It's not mandatory for open water recreational diving the way it is for cave or wreck penetration. On an open water dive, you can ascend at any point. You're not trapped.

That said, it's still a solid habit. Plenty of divers on the Great Barrier Reef have surfaced further from the boat than planned because of current. Having a third of your tank in reserve means you can swim back on the surface or wait for pickup without stressing about air.

On drift dives off Cairns, current does the work and air management is less of an issue. But on wall dives or site-specific dives where you return to a fixed point, the rule of thirds is worth following.

How It Applies to Different Dive Types

Cave Diving

Non-negotiable. The rule of thirds is the absolute minimum for cave diving. Some cave divers use a more conservative rule of fourths or even sixths depending on the system. In Australia, cave diving has strict requirements and isn't something you do without proper training and certification.

Wreck Penetration

Same principle. Once you're inside a wreck, you need air to get back out. The Queensland Parks and Wildlife Service notes that wreck diving requires careful planning and experience. The Ex-HMAS Brisbane off the Sunshine Coast is a popular wreck dive that sits in 28 metres of water, and all divers need a recognised scuba qualification and a permit to dive it.

On the GBR, the Great Barrier Reef Marine Park Authority manages heritage shipwreck sites with special management areas. Dive planning and air management are part of diving these sites responsibly.

Open Water Reef Diving

Most recreational dive operators in Cairns use a simpler version: surface with 50 bar minimum. That's the standard briefing on day boats heading to the outer reef. The rule of thirds gives you more margin, especially on deeper dives or dives with any kind of swim-back to the mooring line.

Shore Diving

If you're doing shore dives around FNQ, the rule of thirds is good practice. You're swimming out from shore, so you need air to get back. No boat to pick you up if you surface a couple of hundred metres offshore with an empty tank.

How to Calculate Your Turn Pressure

Quick maths. Take your starting pressure. Divide by three. Subtract that from your start pressure. That's your turn point.

  • Start: 200 bar. Third = 67 bar. Turn at 133 bar.
  • Start: 230 bar. Third = 77 bar. Turn at 153 bar.
  • Start: 180 bar. Third = 60 bar. Turn at 120 bar.

Check your dive computer or SPG regularly. Some computers have configurable gas alerts you can set to your turn pressure. Much easier than trying to remember a number when you're watching a turtle cruise past.

Air Consumption and Why It Matters

The rule of thirds assumes equal air usage on the way out and back. That's a simplification. In practice, you'll often use more air on the outbound leg because you're going deeper, working against current, or settling into the dive. Coming back is usually shallower and calmer.

That works in your favour. If you use more than a third going out and less than a third coming back, you still end up with your reserve intact.

What matters more than the exact maths is knowing your own air consumption. Some people burn through a tank in 40 minutes. Others get 70 minutes from the same tank. A dive computer with air integration gives you real-time data on your consumption rate and remaining time. That's the best tool for air management, with the rule of thirds as a backup framework.

Buddy System and Shared Air

Part of the reason the reserve third exists is for buddy emergencies. If your dive buddy has an out-of-air situation, they're breathing from your tank via your alternate air source. Now two of you are sharing one tank's remaining air.

Without a reserve third, that scenario gets dangerous fast. With one, you've got enough gas for both of you to make a controlled ascent or swim to the exit.

The Queensland Recreational Diving Code of Practice 2024 requires commercial dive operators to have procedures for managing diving emergencies. For private divers, the buddy system and proper air management are your first line of defence.

Queensland Dive Safety

Queensland regulates recreational diving through the Safety in Recreational Water Activities Act and the 2024 Code of Practice. Commercial operators running trips out of Cairns, Port Douglas, and the Whitsundays must comply with these regulations, which cover dive planning, gas quality, decompression management, and emergency procedures.

WorkSafe Queensland's dive risk assessment guidance specifically mentions that divers should carry bail-out bottles and reserve air for deeper or more complex dives. The rule of thirds aligns with this approach to air management.

For private divers, there's no legal requirement to follow the rule of thirds. But every training agency teaches air management as a core skill, and the rule is the simplest framework for getting it right.

The 1/3 Rule vs Other Air Management Rules

Rule of thirds: One third out, one third back, one third reserve. Standard for overhead environments.

50 bar rule: Surface with at least 50 bar remaining. Common on recreational reef dives. Simpler but doesn't account for distance from exit.

Turn on half: Turn the dive when you've used half your air. More conservative than thirds for open water. Good for shore dives.

Rule of fourths: Used by some cave divers. One quarter out, one quarter back, two quarters reserve. More conservative than thirds.

For most recreational diving around Cairns and the GBR, the 50 bar rule combined with awareness of the rule of thirds is enough. If you're getting into wreck penetration or cave diving, thirds is the minimum.

Gear That Helps

Good air management starts with the right gear. A reliable regulator that breathes easily at depth makes a difference. A dive computer with air integration tells you exactly how much gas you have and how fast you're using it. And a properly serviced BCD means you're not fighting buoyancy and wasting air.

We service regulators, BCDs, and dive computers at our Westcourt store. If your gear hasn't been serviced recently, it's worth getting done before your next trip.

Other Dive Rules Worth Knowing

The 120 rule: Subtract your depth in feet from 120 to estimate maximum bottom time without deco stops. A different kind of dive planning rule.

Safety stop: Three minutes at five metres at the end of every dive. Not always mandatory but always smart.

Ascent rate: No faster than 9-10 metres per minute on most dive computers. Too fast and you risk decompression sickness.

Not sure what some of these terms mean? Check our diving and spearfishing glossary.

Scuba Gear at Ocean Universal

Whether you need a new regulator, a dive computer with air integration, or just a service before your next trip, we've got it sorted. Pop into our Westcourt store and have a chat. We've been helping Cairns divers for over 25 years.

Ocean Universal
Shop 1/310 Mulgrave Road, Westcourt, Cairns QLD 4870
(07) 4036 2080
www.oceanuniversal.com.au

Formerly Spearfishing Superstore. Built for Divers, BY DIVERS.

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