What Size BC Wing Should I Buy?

When diving, your wing needs only enough lift to compensate for the weight of your breathing gas plus the compression of your exposure suit at depth. Those using suits which don't compress much, such as dive skins or drysuits made of trilaminate or crushed neoprene, will also want some extra lift in order to be able to float comfortably on the surface. And of course the whole rig needs to float by itself if you doff it at the surface.

The wing is used to control buoyancy throughout the dive; a drysuit (if worn) should only be inflated enough to keep warm (please see Dynamic Instability for more information). At the beginning of the dive add enough gas to the wing to offset the weight of breathing gas in your tank; as you breathe the tank down, gradually vent small amounts of gas from the wing to stay neutral. At the end of the dive, when you are at 10ft, if your tank is nearly empty there should be little or no gas left in the wing.

Buying more lift than you need offers no advantages and may cause other problems. Larger wings tend to cause more drag underwater and it can be more difficult to dump gas while ascending. Additionally, some divers with high capacity wings tend to overweight themselves and then try to compensate by adding a lot of gas to the wing; this makes buoyancy control difficult due to dynamic instability and creates a dangerous situation if the wing fails. Additionally some manufacturers which create large capacity wings put on bungie straps to keep these large wings from going all over the place. Please see the note at the bottom about bungied wings (also known as bondage wings).

Those who use integrated weights, such as the Halcyon ACB weights or various types of tank weights, may need a little extra lift. For example, if you have a single steel tank, steel back plate, canister light, argon system, and 14lb of integrated weight then 36lb lift is probably not enough to keep the rig from sinking if you doff it at the surface. The Halcyon single tank Pioneer wings have a unique expanding panel feature that allows the higher capacity models to have no more drag than the 27lb lift model. Another solution is to just use a standard weight belt and avoid the problems associated with integrated weights.

Finally it should be noted that actual lift is not always equal to the rated lift. Often there are portions of the wing that are "pinched off" when a rig is fully assembled which will reduce its effective lift capacity. Therefore it is strongly recommended to either test a wing with your gear, or ask for references from people that have similar gear configurations before making a final decision.

Let's consider a few practical examples.

Single aluminum 80, two-piece 6.5mm wet suit:
The breathing gas weighs 6lb or less, and the wet suit could lose up to 24lb of lift due to compression if you go deep enough. So a 30lb wing should be adequate. Note that the 30lb should be sufficient to float the rig at the surface, unless there is significant weight worn integrated with the BP/harness.
Single aluminum 80, tropical dive skin:
The breathing gas weighs 6lb or less, and we would like another 10lbs for surface flotation, so any wing over 16lbs is enough. Note again that 16lb should be sufficient unless there is significant integrated weight (which is unlikely with a dive skin).
Single low pressure steel 95, trilaminate drysuit:

The breathing gas weighs 7lb or less, we would like at least 10lbs for surface flotation (more for rough conditions), and drysuit divers often carry a little extra weight for warmth. Then we must consider that a steel 95 is at least -2lb negative (some makes are more negative), a steel backplate is -6lb and the canister light is usually at least -2lb. A 27lb wing might be sufficient for this configuration and some divers have successfully used a 27lb with this configuration. However more divers opt for having a little more margin and choose the 36lb wing, which should be more than sufficient for most applications. If significant amounts of integrated weights and/or non-ditchable weights are used a 45lb wing may be more appropriate.
Double low pressure steel 104s, two aluminum 40 stage tanks, trilaminate drysuit:
The breathing gas could weigh up to 27lbs (although probably less due to using helium mixes). For the sort of dive where this equipment configuration would typically be used we want to be able to maintain neutral buoyancy without dropping any weights even if the drysuit completely floods (loses all of its lift). Most divers find that a wing in the 55lb range works well.

We don't need extra wing lift to bring up a heavy object from the bottom; this is dangerous since if you drop the object the extra gas in your wing will send you rocketing toward the surface. It is safer to use a lift bag instead.

Although it is possible to use a double tank wing with a single tank and adapter this can cause a lot of drag and gas trapping and is not a recommended practice. Always use the correct gear for the dive.

Bungied Wings

The use of bungied/bondage wings is strongly discouraged. To start with, one of the primary reasons stated for their use is that they streamline your rig. Ironically, they generally do the exact opposite. Hydrodynamics dictates that rough surfaces create increased turbulence which consequently increases drag. The bungies create a very rough surface and thus are adding to drag. Furthermore the bungies have a tendency to trap air which cause both static and dynamic instability issues. However these issues are not the most important reasons to avoid these wings. There are two large reasons that bungied wings are normally avoided. Probably the largest problem with bungied wings is the increased resistance to oral inflation. The bungies will make it significantly more difficult to orally inflate the wing, which can be a serious safety issue. A related issue is that the bungies create a stronger positive-pressure deflate than normally exists. So deflating the wing tends to dump air much faster than a non-bungied wing. This makes proper buoyancy control more difficult. And finally, due to this positive-pressure deflate, it is nearly impossible to use your BC as a "third regulator". This is somewhat of an advanced topic, however your BC can be used as an alternate regulator in very serious conditions. The deflate and inflate buttons are depressed simultaneously to provide air and then the diver breaths out their nose, or removes the BC inflator from their mouth when exhaling. It is important to note that you are not rebreathing the air in the BC, you are effectively breathing it straight from the BC inflator mechanism. Obviously you will want to practice this skill in a pool before trying it "for real"!