As a new diver you are taught to plan the dive so that you use one third of your air on the way out, one third on the way back so that you leave the water with one third as a reserve for use during the dive if you need to sort out a problem.
When you are learning in shallow water with a direct rout to the surface this may seen over kill, but you are being prepared for your later career in diving. As the diving bug bites you may want to try wreck or cave diving, these type of dives often mean there is no direct rout to the surface.
The rules of thirds come from technical cave diving and depending on whether you are reading an English or American blog this is attributed to either UK cave divers from the Cave Diving Group (CDG) or American counterparts exploring the springs of North Central Florida.
Cave divers allow for one third of the gas supply to be used on trip in to the cave, one third for the dive out and a third held in reserve in case things did not go according to plan.
When planning our gas, rules of thirds we have to consider that we may not all breathe at the same rate, we may not all have the same size cylinders. When we plan we will always agree to turn the dive when the first person reaches the turn pressure first.
If we have a gas fill of 210bar the math’s is easy 210 divide by three is 70. So the first third is used when we get to 140bar.
With a gas fill of 200 the math’s is a tad more difficult, but if we round down to 95bar and divide this by 3 we have 65bar. Two hundred minus 65, equals 135bar.
Same with 190bar fill round down to 180bar, divide this by three equals 60bar, means we will turn the dive when we reach 120bar.
The fills of 200 and 190 bar all are really good examples of conservative diving, as the rounded down gas is included in your reserve gas. Should we have a problem at our turn point this additional gas means we have time to sort the problem and turn the dive.
As a conservative diver we have learnt not to dive to extremes, the same applies with rules of thirds turn within the first third not on the limit of it. If you are a new diver or you use air faster then your buddy do not let this stop you turning the dive you are not ruining it for anyone, having you come out with no air will truly ruin it.
If you are not sure with your math’s talk the sums through this means that you dive buddies will help and correct you if you get it wrong.
Andy Torbet has put a great video on facebook that I think makes this very clear and explains this for a self sufficient diver using side mount, as a back mount buddy the rules allow you and your buddy to share the gas in one cylinder to exit the dive safely. https://www.facebook.com/OfficialAndyTorbet/videos/235216940497947/