Dive weighting: new cylinder or water
Changing cylinder or water? Enter your usual cylinder and weight, then the new cylinder: the tool tells you how much lead to add or remove.
What you know
The cylinder you are well weighted with, and that weight.
Cylinder
Adjust this cylinder ▾
Water
Your weight with this cylinder
What you will dive
The new cylinder, and the water you will dive in.
Cylinder
Adjust this cylinder ▾
Water
Settings
Most French clubs aim for 50 bar at the end of the dive; manufacturers’ data sheets often give buoyancy at 35 bar.
Add 2.5 kg
at sea8.5 kg
Exact value: 6.0 kg + 2.3 kg = 8.3 kg, rounded to 0.5 kg.
Theoretical result: do a weight check
This calculation gives a starting point, not your exact weight. Cylinder weight varies from one manufacturer to another, even for the same capacity: trust the tare stamped on the shoulder, and always test your weight in the water: at the surface before the dive, then at the safety stop at the end (steps below).
Buoyancy of the cylinders alone
| Usual | New | |
|---|---|---|
| full | −3.6 kg | −0.7 kg |
| at 50 bar | −1.1 kg | +1.2 kg |
| Gas used during the dive | 2.5 kg | 1.9 kg |
| Water | sea | sea |
Positive: the cylinder pulls towards the surface. Negative: it pulls towards the bottom. It gets lighter during the dive as you breathe its gas.
Test your weight before the dive
At the surface, BCD empty, lungs full and breathing calmly: you should float with the water at eye level. Breathe out fully: you should start to sink. If not, adjust before you go down.
Check your weight at the end of the dive
- 1Stop at the safety stop, around 3 to 5 m, with about 50 bar in the cylinder.
- 2Empty your BCD completely and breathe normally.
- 3You should hold the stop effortlessly, lungs half full.
- 4Rising? Add 0.5 to 1 kg. Need to keep air in your BCD? Remove the same.
Write down the right weight for this cylinder: it will be your starting point next time.
How it is calculated
Cylinder buoyancy (in kg, positive = it floats) = mass of water displaced − mass of the cylinder.
Displaced volume = empty weight ÷ metal density + water capacity + valve volume (brass, density 8.5). The closed cylinder also displaces the volume of gas inside it.
Cylinder mass = empty weight + valve + gas. Gas mass = capacity × pressure × gas density ÷ Z, with 1.21 g/L per bar for air at 15 °C and Z the compressibility factor of air (≈ 1.02 at 200 bar, 1.09 at 300 bar).
Densities: steel 7.85; aluminium 2.70; fresh water 1.000; sea (Channel, Atlantic) 1.025; very salty sea (Mediterranean, Red Sea) 1.029.
Adjustment = buoyancy of the new cylinder − buoyancy of the old one, both compared at reserve (end of dive, lightest cylinder). Positive: you need to add lead.
Change of water: your body and gear displace roughly their own weight in water. Going from fresh water to the sea (2.5 % denser) gives you about 2.5 % of that weight in buoyancy: equipped weight × (new density ÷ old density − 1).
If you do not know your weight: a classic starting point at sea with an 11 L alu cylinder, as a percentage of your body weight: 2 % with no suit, 5 % in 3 mm, 8 % in 5 mm, 10 % in 7 mm, 10 % + 4 kg in a drysuit. The tool then corrects it for your cylinder and your water. It is an estimate: a weight check is still essential.
Check: the model matches published data sheets within 0.4 kg (Luxfer S80 and S40, Faber 12 L steel).
The suggested weight is rounded to the nearest 0.5 kg. Preset cylinder weights come from manufacturer and dealer data sheets (Luxfer, Faber): the real tare is stamped on your cylinder’s shoulder. Always do a weight check at the start of a dive.
Understanding weighting
Why your weight changes with the cylinder
Cylinders do not have the same buoyancy depending on their size and metal. A 12 L steel cylinder clearly sinks, while an 11 L alu becomes almost neutral, even positive, at the end of a dive. Switch from one to the other without changing your weight and you will be too light or too heavy at the safety stop. The tool compares the buoyancy of both cylinders at reserve, when the cylinder is lightest, and tells you how much lead to add or remove.
Fresh water, sea, Mediterranean
Sea water is about 2.5 % denser than fresh water, and the Mediterranean or Red Sea a little more. The denser the water, the more it holds you up: going from a lake to the sea, you need to add lead, about 2.5 % of your equipped weight. The tool only asks for that equipped weight if the water changes.
Gas has weight
A 12 L cylinder filled to 230 bar holds more than 3 kg of air. As you breathe it during the dive, the cylinder gets lighter: that is why weighting is set for the end of the dive, at reserve. To estimate how much you will use on a dive, try the air consumption calculator. With nitrox, enter its O2 % in the settings: it is slightly heavier than air.
A weight check is still essential
The calculation gives a good starting point, but every diver is different: build, a worn suit, BCD, breathing. Always do a weight check at the end-of-dive stop, at reserve, BCD empty, and adjust by 0.5 to 1 kg. Then write down the right weight for that cylinder: it will be your reference next time.
Frequently asked questions
How much lead going from a 12 L steel to an 11 L alu?
With the default settings (50 bar reserve, at sea), add about 2.5 kg: the alu cylinder floats much more than a steel one at the end of the dive.
Do you need more lead at sea than in a lake?
Yes: sea water is about 2.5 % denser than fresh water and holds you up more. Count about 2.5 % of your equipped weight extra, i.e. 2 to 3 kg for most divers.
How do you estimate your weight if you do not know it?
A common rule, at sea with an 11 L alu cylinder, gives a percentage of your body weight: 2 % with no suit, 5 % in 3 mm, 8 % in 5 mm, 10 % in 7 mm, 10 % + 4 kg in a drysuit. The tool corrects it for your cylinder and water, but only a weight check gives your real weight.
How do you check your weight?
At the end of the dive, at the safety stop around 3 to 5 m with about 50 bar in the cylinder: empty your BCD and breathe normally. You should hold the stop effortlessly, lungs half full. If you rise, add 0.5 to 1 kg; if you have to keep air in the BCD, remove the same.