Fat tissue has a density of roughly 0.9 g/cm³ — slightly less than water, so it floats. Lean mass (muscle and bone) has a density of around 1.1 g/cm³, meaning it sinks. This is why two divers at the same total weight but different body compositions need different amounts of lead.
A diver with higher body fat is more positively buoyant and needs more weight to achieve neutral buoyancy. A lean, muscular diver is already closer to neutral and needs less lead. Ignoring body composition is why so many divers end up over- or under-weighted on their first dive in unfamiliar gear.
Wetsuits are generally made out of Neoprene, a highly buoyant material. A 7mm wetsuit can be 12 to 15 pounds positively buoyant. You need to offset this. Whereas a 3mm to a 5mm suit is generally 4 to 6 pounds.
Aluminum 80s become positively buoyant when empty — as you use your air the tank gets lighter and you become more buoyant toward the end of the dive. Steel tanks stay negatively buoyant throughout, which is why many experienced divers prefer them. If you're diving an aluminum 80, factor in 2 to 4 extra pounds compared to a comparable steel tank.
Salt water is denser than fresh water, meaning you're more buoyant in the ocean than in a lake or pool. If you've only dived in fresh water and are heading to the ocean for the first time, expect to add 2 to 3 pounds.
Newer divers tend to over-weight themselves as a crutch for poor buoyancy control. The calculator adds a small buffer for beginners. As your technique improves, you'll likely find you need less weight over time.
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