Find the right amount of lead: surface buoyancy check with an empty BCD, starting values by suit thickness and fine-tuning in small steps, with a buddy.
How much lead do I need for diving? The honest answer: enough that with an empty BCD and a normal breath you float at eye level, and sink slowly as you exhale. That is the buoyancy check. It takes five minutes and saves air and nerves for the whole dive.
The Divers Alert Network (DAN) names two common mistakes: too much lead and not re-adjusting when suit, cylinder or water change [1]. An overweighted diver pumps more air into the BCD, hangs more upright and creates more drag. An underweighted diver tires from staying down [1]. In a study of 959 recreational dives, buoyancy problems were the most commonly reported difficulty at 45 % [4]. And a clinical reference notes: ineffective buoyancy increases air consumption, fatigue and injuries [3].
The check is a fixed part of training, for example in the US agency NOAA's checklist: "Buoyancy check and weight adjustment" in water shallow enough to stand [2]. This guide explains and motivates, it does not replace an instructor.
Put on the suit you actually want to wear and don the whole unit. The cylinder matters: an aluminium cylinder gets lighter as it empties and floats more at the end of the dive than at the start. The check accounts for exactly that [1].
As a starting point DAN names about 10 % of body weight for a 6 mm suit and about 5 % for a 3 mm suit [1]. At 80 kilos that is roughly 8 and 4 kilos. These are only starting values: drysuits and thick neoprene need more, aluminium cylinders more than steel, salt water more than fresh [1].
Go to water where you cannot stand, release all air from the BCD and breathe in normally. You should float at eye level [1]. Do not inhale deeply, do not hold your breath.
Exhale calmly. You should sink slowly until the water stands just above your head. Breathe in normally again: you rise back to eye level [1]. Milestone: your body rises and falls with your breath without you paddling.
Change lead in steps of about two pounds (roughly 1 kilogram) and retest [1]. Do not change everything at once: one change, one check.
DAN describes a test for correct weighting: with about 500 psi (roughly 35 bar) in an aluminium cylinder and an empty BCD, you must be able to hover effortlessly at around 15 feet (about 4.5 metres) [1]. That is your safety stop depth. If you need air in the BCD for this, you lack lead.
Keep a lead logbook. For every new setup, write down suit, cylinder, water, lead and the result of the check. After five entries you know your pattern. And you avoid the classic: on holiday simply taking the group's lead because that is how it is done there. Many overweighted divers do not even realize it [1].
As a starting point about 10 % of body weight with a 6 mm suit and 5 % with 3 mm [1]. Then you check with an empty BCD and a normal breath: you float at eye level. Everything else is fine-tuning.
You constantly pump air into the BCD, hang upright and use more air [1]. At the end of the dive you are not missing lead, you already carry too much of it.
Yes. Salt water is more buoyant, so you need more lead there than in a lake [1]. Re-check after switching between waters.
About one kilogram is accurate enough in practice [1]. What matters is that you repeat it whenever suit, cylinder or water change.
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