How much lead do you need? A starting point and a weighting check

The starting point

Loggerfish starts from a share of your body weight for the suit: about 5 % in a 3 mm wetsuit, 8 % in 5 mm and 10 % in 7 mm. A drysuit starts at 10 % and adds a little for the air inside it, more with a thick undersuit. Salt water lifts you about 2.5 % more than fresh, so fresh water takes some off. A steel cylinder is heavier than aluminium, so it takes some off too.

For a 75 kg diver in a 5 mm wetsuit with an aluminium cylinder in salt water, that is 8 % of 75 kg: 6 kg. The same diver in a lake needs about 4 kg.

Try it with your numbers

The calculator starts on the 75 kg example. Change the suit, the cylinder or the water and watch the steps behind the total.

The check that matters

The number is a start, not an answer. Check it:

1. At the surface, with an empty BCD and a normal breath held, you should float at eye level, and sink slowly as you breathe out.
2. At the end of the dive, with little gas left, you should hold a stop at 5 m without effort.
3. If not, change the lead by 1 kg and check again.

The second check is the one people skip. A cylinder gets lighter as you breathe it, and aluminium cylinders turn buoyant when nearly empty.

What the number does not tell you

• It does not know your body, your wetsuit's age or your BCD. Two divers of the same weight can need different lead.
• It does not replace a buoyancy check with an instructor, especially in a new suit or with new kit.

These calculations are planning aids, not a dive computer. Your training and your computer decide the dive.