The Rule of Thirds: It’s not what you think it is

Rule of ThirdsThe reason cave divers follow The Rule of Thirds is one of those things that, supposedly, “everybody knows.” It’s to help ensure that you have sufficient gas to share air with another diver from your maximum point of penetration, all the way back to the cave entrance…right? Well, not exactly.

The Rule of Thirds came into widespread use over 40 years ago when explorers such as Sheck Exley argued that Thirds provided a safer approach to gas management than the once-common Half Plus 200 rule, an approach that had resulted in a number of diver fatalities. As a cave diver, it’s important you understand all of the reasons you might need that additional third, why it works — and when and why you might want to allow an even greater safety margin. Let’s look at each of these in greater depth.

Loss of visibility

Many cave instructors start their courses with an exercise in which they have students time themselves while walking next to a guideline on dry land. Then they have students repeat the exercise, this time with eyes closed and one hand on the line. What students discover is that it typically takes at least twice as long to navigate the course when you can’t see as it does when you can. And twice as long means you will need twice the air. (But wait — wasn’t that extra air supposed to be for your buddy? Whoops!)

Lost guideline

In a separate article, we explain how you can use The Rule of Thirds when determining how much air you can safely allocate for searching for lost team members. But what if it is you — or your entire team — that is lost off the guideline? It’s impossible to say, ahead of time, how much gas you will need to find your way back to safety; however, by keeping that extra third in reserve, you will have a gas volume equal to what you intended to use to reach your max penetration available to find what we hope is a short distance back to the line. That is, you will unless something else goes wrong. (Again…whoops!)

Entanglement, entrapment and similar problems

Having to search for a lost guideline is by no means the only problem for which you might need additional breathing gas. You or a team mate could become entangled in guideline, stuck in a tight passageway or have to repair a broken guideline or piece of equipment. And these are by no means the only other things that could go wrong. None of these problems can be solved instantly, and the longer it takes to solve them, the more gas you may need.

Gas sharing

Now we come to the issue The Rule of Thirds is supposed to be about. As a cave diver, you go to great lengths to ensure that you always have sufficient gas to get both yourself and a team mate out of the cave from your maximum point of penetration. The catch is, if you really did have to share gas with another diver all the way from your maximum point of penetration to the cave entrance, merely having the same amount of gas as the affected diver needed to enter would most likely not be enough.

Why? At least two reasons come to mind:

  • Both the donor and receiver are likely to be under considerable stress and, as a consequence, breathing a lot harder.
  • It’s difficult to maintain the same speed while sharing air — even in good visibility — as you do when not sharing gas.

(Did we mention the part about “Whoops?”)

Is it enough?

At this point, you may be thinking, “OMG! There’s no way The Rule of Thirds can address all of these problems. Maybe we should be following a Rule of Fourths…or Sixths.”

Relax. Despite the apparent conflicts, the fact remains The Rule of Thirds has proven itself a perfectly reasonable approach to cave diving gas management for over four decades. What is important to understand is why. What it comes down to is that, for every potential shortcoming in The Rule of Thirds, there are generally one or more mitigating factors that offset it. Let’s take a look.

  • Loss of visibility: Is it theoretically possible you could face a zero-visibility situation all the way back to the cave entrance? Of course — just like it is theoretically possible a meteor could hit the Earth and collapse the cave on top of you. Neither is a situation worth losing sleep over. In reality, a zero-vis situation seldom exceeds a radius of 30 m/100 ft. Meaning that, so long as you’ve kept a hand on the guideline, maintain good body postition and proper technique, you can be back to better visibility and exiting the cave in short order.
  • Loss of guideline: Just how far off the guideline are you likely to get? You should have enough line on your safety reels/spools to search a considerable distance in any direction. Given that you have sufficient reserve gas to make it all the way back to the cave entrance, your odds of finding the misplaced guideline are actually pretty good, even if you have to search in more than one direction to do so.
  • Flow: This is a significant factor in caves such as Madison Blue, Telford, Cow, Little River, Devil’s Eye and Manatee. You know from experience that it can take a lot less gas to exit in flow than it does to fight your way into the cave.
  • Multiple team mates: Why are three-person teams ideal? Because you have not one, but two potential donors, all but guaranteeing there will be sufficient reserve gas to bring an out-of-air team mate back from anywhere in the cave.
  • Sidemount: With the increasing popularity of sidemount, we increase the likelihood divers can solve their own gas emergencies without having to rely on team mates.
  • Total gas loss at maximum point of penetration? Could a team mate actually lose all of their breathing gas at your maximum point of penetration? Only if a valve literally popped off a cylinder — in which case the explosive release of gas would likely propel the hapless diver across the cave with such force that when he hit the nearest wall, he’d be knocked unconscious (game over).

Even the most catastrophic of regulator or valve failures cannot drain a set of tanks instantly. We once watched a full set of doubles drain itself following a burst disk failure; it took four minutes. In two minutes you can get a lot closer to the exit and reduce the amount of gas needed to reach the entrance by at least a couple hundred pounds.


In reality, the only time a cave diver is likely to be totally and completely without gas is if he becomes separated from his team mates and hopelessly lost, or if the entire team becomes hopelessly lost. In either case, there will be no one present with sufficient reserve gas to help.

What happens in most gas-sharing situations is that the affected diver is not totally and completely without gas, but rather there is doubt as to whether or not his remaining gas will be sufficient to reach the entrance. Examples would include:

  • A valve or manifold fails and continues leaking uncontrollably.
  • A free-flowing regulator has been shut down, but only after it caused the affected diver to lose considerable gas.

Why does The Rule of Thirds work in these situations?

  • In the case of a valve failure on a set of manifolded doubles, you are going to isolate the cylinders and have the affected diver breathe from the damaged side until it is exhausted. At this point, the gas remaining in the isolated side will, in all likelihood, be sufficient for him to exit the cave without having to share gas.
  • In the case of a free-flowing regulator having caused the loss of a significant amount of gas, the problem will not be that the diver is without air, but rather that there is doubt as to whether the remaining gas will be enough to exit. In this case, there is no need to share air all the way out of the cave. You only need to get the affected diver close enough to the exit that there is no doubt he has sufficient gas to make it the rest of the way, with a reserve.

Despite all the evidence that following The Rule of Thirds will provide a sufficient gas reserve in most situations, there are instances in which it would be wise to remain well within your theoretical turnaround point. These may include:

  • Unfamiliar cave
  • Abundant silt
  • Complex navigation
  • New/unfamiliar equipment

You get the idea. Just as it is foolish to consistently push no-decompression limits or do, without exception, only the minimum required decompression, there are circumstances in which pushing The Rule of Thirds to its absolute limit may be showing poor judgment. As they say, “Let common sense prevail.”

What The Rule of Thirds really means

What many cave divers don’t understand is that The Rule of Thirds was not originally a diving term. It comes to us from the worlds of art and photography, and describes a way to achieve the most visually pleasing compositions. The values represented by The Rule of Thirds are actually somewhat arbitrary. They just happen to work.

In the real world of cave diving, following The Rule of Thirds means more than just dividing your starting pressure by three. It means ensuring that, no matter where you are in the cave, you always have at least twice the gas you theoretically need to exit. This comes into play in any number of situations, including:

  • Searching for a lost team mate
  • Penetrating in two directions on the same dive
  • Doing circuits and traverses

In this context, it may be more helpful to think of it as the Twice as much gas as I need to get the Hell outta here… Rule. Doing so will help you live long…and prosper.


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