You already know the thirds rule. But what if current makes the way back cost more than the way in? The sixths rule, flow, and the math that actually counts.
You already know the thirds rule from our intro guide: one third in, one third out, one third reserve. But the question left open there is the decisive one: what if the way back costs more gas than the way in? Honest answer: then a simple third isn't enough.
Current is the most common reason. Swim against the flow into the cave and you noticeably burn more gas per minute on the way in than on a relaxed return with the current at your back — or vice versa. The thirds rule assumes symmetric consumption; once current is in play, that assumption no longer holds.
For less experienced cave divers there's a stricter variant: the rule of sixths. It limits penetration to a sixth of the gas supply instead of a third — with the explicit goal of leaving a beginner a huge reserve to solve problems with. You're not diving on the edge here, but with a safety cushion almost twice as large as an experienced team's.
Train Smart. Play Hard. in gas planning means, very concretely: calculate for the worse case, not the convenient average.
One third in, one third out, one third reserve — this base framework counts as the global minimum standard. Important: the last third isn't only meant for you — it must be enough, in an emergency, to also get a buddy with a gas problem out.
With the rule of sixths you limit your penetration to a sixth of your total gas supply instead of a third. That sounds conservative — it is, deliberately. It's meant for divers who don't yet have enough cave experience to reliably estimate their own consumption under stress.
Estimate consumption for the way in and out separately, not as an average. Need three times as much gas against the current as with it? You're no longer planning with thirds, but modified quarters or fifths — matched to the pricier leg. Rule of thumb: base your numbers on the less favorable direction, never the convenient one.
Set your turn pressure (the pressure at which you're guaranteed to turn around) in writing or on a wrist slate before you enter the water. Example: with 200 bar total pressure and the rule of sixths, you turn around at roughly 167 bar — calculated, not eyeballed.
Your personal calculation isn't enough — you have to plan around the weakest link in the team. If a teammate has higher consumption or a smaller tank, that value sets the turn point for everyone, not your own.
Gas planning isn't a formality for the dive school — it's the difference between a planned turnaround and an emergency. Whoever does the math underwater for the first time is calculating under stress with less oxygen reaching the brain than on land.
Always plan for the case that a teammate needs to share your gas. The reserve doesn't exist for the ideal case — it exists for the moment current, a gas problem and stress all hit at once.
Write down your turn pressure twice: once on your wrist slate, once in your head as a round number you can recall instantly even under stress. Two redundant reminders beat a perfect calculation you forget in an emergency.
A detail experienced cave divers know: measure your own consumption under real exertion, like swimming against light current, not calm shallow water — that number is the only one that counts in an emergency.
The thirds rule allows a third of your gas supply for the way in. The sixths rule limits that to a sixth — with much more reserve, meant for divers with little cave experience whose own consumption under stress is still hard to estimate.
Because it assumes symmetric consumption in and out. Swim against the current going in and with it coming back, and your consumption per direction differs completely — the calculation must follow the pricier direction, not the average.
Set it before the dive, based on the ratio you're using (thirds or sixths) and the highest consumption in the team. Write the number on your slate instead of recalculating it in your head underwater — stress reliably makes mental math worse.
For the whole team. The turn point always follows the diver with the highest consumption or the smallest tank — your own, more favorable calculation doesn't count if a teammate hits their limit earlier.
Cave Diving: When the Line Is Gone — Emergency Procedures