Weighting and buoyancy

In short

Being correctly weighted means carrying just enough weight to descend easily at the start of the dive and to hold your stop effortlessly at the end, with an almost empty cylinder. In between, you adjust your buoyancy with the BCD and your breathing. The right weight is found with a simple check, written down and checked again whenever your equipment changes. It is the basis of a comfortable dive that saves air and respects the seabed.

Why it matters

An overweighted diver puts a lot of air in the BCD, swims at an angle, gets out of breath and uses more air; they may hit the bottom. An underweighted diver struggles to descend and, above all, may no longer be able to hold the stop at the end of the dive when the cylinder has become lighter: the ascent becomes hard to control. Correct weighting makes diving safer and more pleasant.

Archimedes' principle applied to divers

Any body immersed in water receives an upward force equal to the weight of the volume of water it displaces. A diver floats, sinks or stays neutral depending on the difference between their weight and this force. Several things change buoyancy during a dive:

• the suit: neoprene floats and compresses with depth; you become less buoyant as you go down;
• the cylinder: it gets lighter as it empties, by about 1 kg for every 800 litres of air used; an aluminium cylinder becomes clearly buoyant at the end of the dive;
• the water: denser sea water supports more than fresh water; in a lake, you need less weight;
• the lungs: full lungs make you rise slightly, empty lungs make you sink;
• the BCD: the air inside it compresses or expands with depth.

Finding your weight

The surface check

Fully equipped, in deep water, with the BCD completely empty, breathe normally from your regulator:

• with the right weight, you float at eye level at the end of an inhalation and start to sink as you exhale;
• if you stay at the surface while exhaling, you need more weight;
• if you sink with full lungs, you have too much.

Fine-tuning at the end of the dive

This check is done with a full cylinder, which is heavier than at the end. The real benchmark is the stop: at the end of the dive, with an almost empty cylinder (around 50 bar) and an empty BCD, you should be able to stay level at 3 m. If you have to fin downwards, add a little weight next time; if you have to keep a lot of air in the BCD, remove some.

Writing it down

Write down your weight for each configuration: type and thickness of suit, steel or aluminium cylinder and its volume, fresh or salt water. You will save time and avoid guesswork on every new dive.

Where to carry the weight

• Weight belt: simple and universal; the buckle must open with one hand.
• Integrated weights in the BCD: more comfortable for the back, spread in releasable pockets.
• Trim pockets: small pockets placed high on the BCD to balance a horizontal position; they are usually not releasable.

Whatever the system, you must know how to drop your weights in an emergency, at the surface, to float without effort, and know how your buddy's work. Underwater, dropping weights causes an uncontrolled ascent: it is only a last resort.

Controlling buoyancy

• The BCD compensates for volume changes: add small amounts of air as you descend and release some as you ascend, in small steps, waiting for the effect before doing it again.
• Lung control fine-tunes your balance: a slightly deeper or shallower breath is enough to rise or sink a few tens of centimetres, without ever holding your breath.
• A horizontal position (trim) reduces finning effort and protects the seabed; it depends on how the weight is distributed and how the cylinder is positioned.
• Avoid yo-yo profiles: repeated ups and downs increase the risk of decompression sickness and barotrauma. Anticipate, correct early and in small amounts.

Overweighting and underweighting

Situation	Possible consequences
Too much weight	Very inflated BCD, tilted position, finning effort, breathlessness, higher air consumption, contact with the seabed, harder emergency ascent
Not enough weight	Difficult descent, stop impossible to hold with an almost empty cylinder, uncontrolled ascent

Key takeaways

• Correct weight: level at a 3 m stop, almost empty cylinder, empty BCD.
• Do the surface check, fine-tune at the end of the dive and write the result down for each configuration.
• Suit, cylinder and fresh or salt water change your weighting.
• Know how to drop your weights and your buddy's.
• Adjust the BCD in small steps and avoid yo-yo profiles.

Further reading

The thermal protection guide explains the effect of the suit on buoyancy; the scuba equipment guide presents the BCD and its dump valves; the environment guide shows why good buoyancy control protects the seabed.