How this calculator works
A diver needs enough lead to sink on a full exhale at the end of the dive, but not so much that inflating the BCD becomes a constant struggle. Published starting tables scale the suit allowance with body mass: about 2 percent for a swimsuit or dive skin, 5 percent for a 3 mm wetsuit, and 10 percent for a 5 mm suit or a 7 mm suit plus 2 kg for the thicker neoprene, hood and boots. Salt water is denser than fresh water, so the same diver needs less lead in fresh water. Aluminium cylinders become positively buoyant as they empty, which is why the estimate adds lead to compensate, while steel cylinders do not.
Formula
Suit lead = body mass x suit factor, where the factor is 0.02 for a swimsuit, 0.05 for a 3 mm suit or 0.10 for 5 mm and 7 mm suits. A 7 mm suit adds 2 kg. Water adjustment = body mass x 0 for salt water, -0.0175 for brackish or -0.035 for fresh. Cylinder adjustment is +2 kg for aluminium and 0 for steel. Starting ballast = suit lead + water adjustment + cylinder adjustment, never below zero.
Worked example
A 75 kg diver in a 5 mm wetsuit, salt water and an aluminium cylinder has a suit allowance of 7.5 kg, no water adjustment and 2 kg for the cylinder, giving 9.5 kg of starting ballast. The same diver in fresh water subtracts 75 x 0.035 = 2.6 kg, giving 6.9 kg. In a 7 mm suit the suit allowance rises to 7.5 + 2 = 9.5 kg, so the salt-water total becomes 11.5 kg.
Advertisement
Frequently asked questions
Why weight for the end of the dive?
A full cylinder is heavier than an empty one. If you weight for the start, you will be underweighted and unable to stay down at the end when the tank is light. Test with a cylinder near 50 bar so the check reflects the worst case.
How much difference does fresh water make?
Fresh water is about 2.5 percent less dense than sea water, so a diver needs roughly 2 to 3 kg less lead for the same kit. The exact amount depends on body mass and suit volume, which is why the page scales it with mass.
Do aluminium and steel cylinders really need different lead?
Yes. An aluminium cylinder is close to neutral when empty, and a typical 12 litre tank loses around 2 kg of air weight over a dive, so it needs extra lead. A steel cylinder is negative when empty and needs less.
What if I am still floating at the safety stop?
Add lead in small steps, usually 1 kg at a time, and repeat the buoyancy check. If you are sinking quickly with an empty BCD at the surface, remove lead. A dive professional can help you fine-tune the figure for your kit and body composition.