The scuba diver must be weighted sufficiently to be slightly negatively buoyant at the end of the dive when most of the breathing gas has been used, and needs to maintain neutral buoyancy at safety or obligatory decompression stops.

Surface-supplied divers may be more heavily weighted to facilitate underwater work, and may be unable to achieve neutral buoyancy, and rely on the diving stage, bell, umbilical, lifeline, shotline or jackstay for returning to the surface.

Dropping weights increases the risk of barotrauma and decompression sickness due to the possibility of an uncontrollable ascent to the surface.

The weights are generally made of lead because of its high density, reasonably low cost, ease of casting into suitable shapes, and resistance to corrosion.

There is some concern that lead diving weights may constitute a toxic hazard to users and environment, but little evidence of significant risk.

The primary function of diving weights is as ballast, to prevent the diver from floating at times when he or she wishes to remain at depth.

The weights are used mainly to neutralise the buoyancy of the exposure suit, as the diver is nearly neutral in most cases, and there is little other equipment carried.

As a corollary to this practice, freedivers will use as thin a wetsuit as comfortably possible, to minimise buoyancy changes with depth due to suit compression.

[1] The scuba diver generally has an operational need to control depth without resorting to a line to the surface or holding onto a structure or landform, or resting on the bottom.

This requires the ability to achieve neutral buoyancy at any time during a dive, otherwise the effort expended to maintain depth by swimming against the buoyancy difference will both task load the diver and require an otherwise unnecessary expenditure of energy, increasing air consumption, and increasing the risk of loss of control and escalation to an accident.

An average person has a surface area of about 2 m2,[9] so the uncompressed volume of a full one piece 6 mm thick wetsuit will be in the order of 1.75 x 0.006 = 0.0105 m3, or roughly 10 litres.

A dry suit will also compress with depth, but the air space inside is continuous and can be topped up from a cylinder or vented to maintain an approximately constant volume.

[3][5][6] A diver ballasted by following this procedure will be negatively buoyant during most of the dive unless the buoyancy compensator is used, to an extent which depends on the amount of breathing gas carried.

The extra weight and therefore negative buoyancy at the start of the dive could easily be as much as 13 kg for a diver carrying four cylinders.

[2][10] Examples: Optimum weighting for scuba allows the diver to achieve neutral buoyancy at any time during a dive while there is still usable breathing gas in any of the cylinders carried, using the least amount of ballast.

The lightweight demand helmets in general use by surface-supplied divers are integrally ballasted for neutral buoyancy in the water, so they do not float off the diver's head or pull upwards on the neck, but the larger volume free-flow helmets would be too heavy and cumbersome if they had all the required weight built in.

The US Navy Mk V standard diving system used a heavy weighted belt buckled around the waist, suspended by shoulder straps which crossed over the breastplate of the helmet, directly transferring the load to the buoyant helmet when immersed, but with a relatively low centre of gravity.

Combined with lacing of the suit legs and heavy weighted shoes, this reduced the risk of inversion accidents.

This ability is of great importance for both convenience and safety, and also reduces the environmental impact of divers on fragile benthic communities.

Small errors can be compensated fairly easily, but large offsets may make it necessary for the diver to constantly exert significant effort towards maintaining the desired attitude, if it is actually possible.

Fine tuning of trim can be done by placing smaller weights along the length of the diver to bring the centre of gravity to the desired position.

All or part of the weighting system may be carried in such a way that it can be quickly and easily jettisoned by the diver to increase buoyancy, the rest is usually attached more securely.

Breathhold and scuba divers generally carry some or all of their weights in a way that can be quickly and easily removed while under water.

The technique for shedding weights in an emergency is a basic skill of scuba diving, which is trained at entry level.

Each shot pellet should be coated[clarification needed] to prevent corrosion by sea water, as use of uncoated shotgun shot for sea diving would result in the lead eventually corroding into powdery lead chloride These are stored in pockets built into the buoyancy control device.

Some designs also have smaller "trim pouches" located higher in the BCD, which may help the diver maintain neutral attitude in the water.

Some rebreathers (e.g. the Siebe Gorman CDBA) have a pouch containing lead balls each a bit over an inch diameter.

Surface-supplied divers often carry their weights securely attached to reduce the risk of accidentally dropping them during a dive and losing control of their buoyancy.

[27] Although lead is the least expensive dense (SG=11.34) material available, it is a toxic substance causing biological damage to wildlife and humans.

Stainless steel and tungsten dive weights for example are currently only obtainable by milling down a solid metal stock material in block or cylinder form, into the required shape.