Calculator Inputs
Example Data Table
| Scenario | Body Mass (kg) | Gear Mass (kg) | Suit Lift (kg) | Cylinder | Water | Depth (m) | Net Buoyancy Surface End (kg) |
|---|---|---|---|---|---|---|---|
| Warm-water, AL80 | 75 | 12 | 4 | AL80, 200→50 bar | Salt (35 ppt) | 18 | ≈ +0.4 to +1.5 (depends on volumes) |
| Thicker suit, freshwater | 85 | 14 | 8 | Steel 12L, 210→50 bar | Fresh | 12 | ≈ +1.0 to +3.0 (often needs ballast) |
| Drysuit, heavy steel | 70 | 18 | 10 | Steel 15L, 230→60 bar | Salt | 25 | ≈ -1.0 to +2.0 (trim carefully) |
These rows illustrate typical setups. Real buoyancy varies with equipment shape, undergarments, and exact cylinder model.
Formula Used
- Weight: W = m·g
- Buoyant force: F_b = ρ_water·g·V_displaced
- Net buoyancy (kg lift): B_net = (ρ·V + B_suit + B_cyl) − m_total
- Apparent weight at depth: W_app = (m_total − (ρ·V + B_suit + B_cyl))·g = −B_net·g
- Suit compression: B_suit(depth) = B₀·e^(−k·depth)
- Gas mass estimate: m_gas ≈ (V_water·P_bar)·ρ_gas,surface
Here, “kg lift” is an equivalent mass that represents upward buoyant force divided by g.
How to Use This Calculator
- Choose units, then enter your body mass and either density or volume.
- Add gear dry mass and an estimate of how much volume it displaces.
- Enter exposure suit buoyancy at the surface and a compression factor.
- Select a cylinder preset, then set start and end pressures.
- Pick salt, fresh, or custom water density and specify depth.
- Click Calculate and review surface, depth, and safety-stop buoyancy.
- Use the ballast guidance and export your results as CSV or PDF.
1) Why divers feel “lighter” underwater
Underwater, your apparent weight decreases because water pushes upward with a buoyant force equal to the weight of the displaced water. When buoyancy nearly matches total system weight, you hover with minimal effort. This calculator converts that balance into clear numbers.
2) Displaced volume drives buoyancy
Buoyancy depends on displaced volume: your body plus gear volume. Higher volume at the same mass increases net lift. Small changes matter: an extra 2 liters of displacement can add about 2 kilograms of lift in saltwater, changing trim and fin workload.
3) Fresh versus saltwater differences
Saltwater is denser than freshwater, so it provides more buoyant force for the same displaced volume. Typical seawater near 35 ppt is around 1025 kg/m³, while freshwater is near 997 kg/m³. Many divers need extra ballast when moving from fresh to salt.
4) Exposure suits and depth compression
Neoprene contains gas bubbles that compress with depth. As pressure increases, suit buoyancy falls, making you heavier at depth than at the surface. The calculator models this using an exponential decay factor so you can estimate how much lift you lose at your planned depth.
5) Cylinder buoyancy changes during the dive
Cylinders typically become more buoyant as gas is consumed. Aluminum tanks may start slightly negative and end positive; steel tanks often remain negative but still shift. This tool interpolates between “full” and “empty” buoyancy and also estimates gas mass used from pressure drop.
6) Weighting for the end of the dive
Smart weighting targets neutral buoyancy near the end of the dive at the surface or shallow stop. If you are neutral only at the start, you may struggle to hold a safety stop later. The ballast guidance here is based on end pressure so you can tune weighting safely.
7) Trim, comfort, and energy use
Excess negative buoyancy increases finning effort and gas consumption, while excess positive buoyancy can force constant venting and reduce control. Balanced buoyancy improves posture and reduces task loading. Use the surface, depth, and safety-stop outputs to spot where instability is likely.
8) Using realistic inputs for reliable results
Measure what you can: body mass, cylinder type, start and end pressures, and an exposure suit estimate. For volumes, begin with reasonable approximations and refine after a real-world weight check. Exporting CSV or PDF helps you compare sessions and keep consistent configuration notes.
FAQs
1) What does “kg lift” mean in the results?
It is buoyant force expressed as an equivalent mass. One kilogram of lift equals the upward force that would balance one kilogram of weight under gravity, making comparisons easy.
2) Why do I get different buoyancy at depth?
Exposure suits compress with depth, reducing their buoyancy. The calculator models this with an adjustable compression factor, so deeper dives typically show more negative buoyancy.
3) How accurate is the gas mass estimate?
It uses cylinder water volume, pressure, and a surface gas density. It is a practical approximation for planning. Real values vary with temperature, gas mix, and cylinder specifications.
4) Should I weight for the start or end pressure?
Most divers weight for the end of the dive so they can hold a shallow stop with a nearly empty cylinder. The guidance is based on end pressure for safer control.
5) How do I choose body density if I don’t know it?
Start with 985 kg/m³ as a reasonable middle value. If you tend to float easily, your effective density may be lower; if you sink readily, it may be higher. Refine after a weight check.
6) Why does changing water type shift ballast needs?
Denser water produces more buoyant force for the same displaced volume. Saltwater usually increases lift, so moving from freshwater to seawater often requires additional ballast to stay neutral.
7) Can this replace an in-water weight check?
No. It supports planning and comparison, but real conditions vary. Use the results to narrow the range, then confirm with a controlled weight check in shallow water.
Notes for Practical Use
- Net buoyancy near zero at the end of the dive improves control.
- Neoprene buoyancy often drops with depth due to compression.
- Cylinders become more buoyant as gas is consumed.
- Do a controlled weight check in shallow water for confirmation.
Dive smarter, tune buoyancy, and record every calculation easily.