The Rule of Thirds in Diving: A Simple Guide for Safer Dives on Koh Tao
Good gas management is one of the most important skills you can develop as a diver, and at Master Divers on Koh Tao, we teach every student the foundations of safe, confident dive planning. One of the most useful techniques is the Rule of Thirds. It’s a straightforward method that helps you plan your air supply so you can enjoy your dive, look after your buddy and return to the boat or shore with a healthy safety margin.
Whether you’re exploring Koh Tao’s beautiful reef systems, heading out on a deep dive, or preparing for more advanced training, understanding the Rule of Thirds will set you up for safer and more relaxed underwater adventures.
What Is the Rule of Thirds in Diving?
The Rule of Thirds is a gas-management strategy that divides your air supply into three equal portions:
- One-third for the outward part of the dive
- One-third for the return
- One-third as a reserve
This means you begin your turn when you’ve used the first third of your gas supply. The final third stays untouched as your safety backup.
At Master Divers, we use this rule when teaching divers how to plan for dives where navigation, depth or conditions require careful gas control.
Why the Rule of Thirds Matters
1. Conditions Can Change
While Koh Tao is known for calm, warm water, factors like currents, visibility and diver workload still play a role in gas consumption. Keeping a full third as reserve ensures you have enough air to manage unexpected changes without stress.
2. Ideal for Deep and Advanced Diving
Sites like Chumphon Pinnacle, Southwest, and the HTMS Sattakut often involve deeper profiles or specific navigation. Using the Rule of Thirds helps divers stay within a safe gas plan while enjoying these sites.
3. Supports Strong Buddy Awareness
At Master Divers, we emphasise team diving. Planning gas based on the diver with the highest air consumption allows the entire team to stay aligned and close enough to help each other if needed.
4. Builds Confidence Underwater
Good gas planning means you’ll surface with extra air, move at a comfortable pace and complete your safety stop without pressure. It’s one of the simplest ways to become a more relaxed diver.
How to Apply the Rule of Thirds
If you start your dive with 200 bar:
- One-third outward: use 65 bar
- One-third return: 65 bar
- One-third reserve: 65 bar
In this example, you begin heading back when your SPG reads 135 bar.
On Koh Tao, this might mean turning the dive at the halfway point of a swim across Twins, adjusting navigation around White Rock, or planning a return route from the Sattakut wreck to the reef.
And remember: your team’s gas plan is based on the diver with the lowest starting pressure or the highest air consumption.
When the Rule of Thirds Is Most Useful
While you may not need the Rule of Thirds on every shallow reef dive, it becomes extremely valuable for:
- Deep dives
- Wreck dives
- Navigation-heavy dive sites
- Drift or current-influenced dives
- Any dive where surfacing in the wrong place is not ideal
As divers progress through the PADI Advanced Open Water or PADI Deep Diver courses with us, this method becomes an important part of safe dive planning.
Common Mistakes Divers Make
- Forgetting to plan around the buddy with the fastest air use
- Not adjusting gas plans for currents or higher workload
- Applying thirds on short-gas dives where the reserve is too small
- Turning too late and leaving no margin for unexpected events
Our instructors coach divers through real-world scenarios so they build confident, repeatable gas-management habits on every dive.
Master Divers’ Approach to Gas Planning
At Master Divers, safety and calm diving are at the heart of everything we do. We teach gas-management skills from your very first training dives, helping you understand why planning matters and how to apply it to Koh Tao’s unique dive sites. The Rule of Thirds is one of many tools we use to help divers make smart, confident decisions underwater.
Whether you’re fun diving, taking your next PADI course or preparing for professional training, learning proper gas management ensures safer, more enjoyable dives around our island.
