When a diver is underwater one of the things he or she watches most closely is the gas gauge because no matter how much fun a person is having on a dive, when the gas is getting low the dive is over. In most recreational diving, air usage is not a problem. It's a straight up and down and if the dive isn't deep, then everything is relatively straightforward. However, there are many other types of diving that require a diver to be in a position in which running out of gas would be certain death. These types of dives include deep diving, cave diving, and wreck diving.
When divers dive in extreme situations, this is generally called technical diving and many types of technical diving require the diver to take extra precautions. One of these is to ensure that they will have enough gas to go in and come back out. Take cave diving for example. A diver has mapped out the route and knows how far it will be to go in and come back out. Now the diver must calculate how much gas will be needed for the journey. If you are diving in this type of situation, then you know you will need a certain amount to go into your dive and you will need the same amount to get out. That is very straightforward. However, what if something goes wrong on your dive? What if it takes you longer to get out than you thought it would? What if something happens to your buddy's air supply and you have to share?
To ensure that there is extra air, that “just in case” air, which might be needed on the dive, technical divers came up with what is now well known as the rule of thirds. This rule states that there is 1/3 going in, 1/3 coming out, and 1/3 that is reserve in case of an emergency. This rule was originally invented by cave divers who were exploring the Florida cave system, a system that has a high outflow making exit easier than entry. For this reason, when following the rule of thirds in other cave systems and other types of diving, it is necessary to allow for more than the reserve gas that is required by the rule of thirds.
Where the rule of thirds goes awry is when divers who are diving together have different sized cylinders or a different rate of breathing. This means that your buddy might be carrying less gas than you are or is using it up faster than you are. When this is the case, you will have to adjust the amount of your emergency reserve so that it is equal to that of your buddy's even if you have more gas in your cylinder. This means that you will have to shorten your trip or, if your buddy has cylinders that are comparable in size, but a higher breathing rate, he or she will have to carry more gas than you carry.
It is also important to consider the rule of thirds even when just recreational diving. Having extra gas in your cylinder never can hurt and if you are doing a simple recreational dive and end up going out longer or deeper than originally intended and if you need to include decompression stops in your ascent, then you will need that extra bit of gas in order to make it back to the surface safely. Besides, even in simple recreational dives, emergencies can happen, whether they are getting tangled in a net, sea grass, or other substance or you have to take the time to help another diver. No matter what, extra gas is a good thing.
As you can see, when it comes to the rule of thirds it is more of a guideline than anything else and a minimum one at that. Consider it a starting point from which you can plan your gas usage during your dives and ensure that you have a reserve that is at a very minimum 1/3 of your total gas. If you are diving alone or if you know you are diving with a buddy who has a lower volume of gas than you or breathes at a faster rate, make the appropriate adjustments to your dive plan so that you don't get stuck without enough air to make the return trip.