Planning tool
Lift-Bag Buoyancy Calculator
Required buoyancy, bag combination from our 100 kg – 50 t range, and free-air volume at depth — the standard pre-plan for salvage and underwater lifting.
| Net lift requirement | 2.50 t |
|---|---|
| Design buoyancy incl. safety factor | 3.75 t |
| Recommended combination | 1 × 5 t bag (Cylindrical (enclosed), Ø150 × 300 cm) |
| Combination total capacity | 5.0 t |
| Free-air volume required at depth | 8.0 m³ |
| Air cylinders (50 L / 200 bar) | ≈ 1 |
| Estimated inflation time | not published for this size |
Indicative pre-planning figures only — every real lift requires an engineered lift plan. Webbing rated to a nominal 7:1 safety factor; PRV set at 15–25 kPa. Final bag combination, drawings, and quotation are confirmed by our engineering team.
How the calculation works
The calculator runs the same three steps our engineering team runs when a salvage customer asks for a bag proposal. First, net lift: the object’s weight in air, minus the weight of the water it displaces (its volume × water density), is the force the bags must actually provide. A safety factor — 1.5 by default — is then applied to cover dynamic loads, unknowns in the weight estimate, and the behaviour of the bag train during ascent.
Second, air supply. Air compresses under pressure: at 12 metres the absolute pressure is roughly 2.2 bar, so each cubic metre of displaced volume at depth corresponds to about 2.2 cubic metres of free air at the surface (the Boyle’s-law approximation of one atmosphere per 10 metres of seawater). The tool converts required displacement into free-air volume and estimates 50 L / 200 bar cylinders at roughly 10 m³ of free air each.
Third, bag selection. The required buoyancy is matched against our published range — parachute, cylindrical, and pillow bags from 100 kg to 50 t — choosing the combination with the fewest bags, and within that, the smallest total capacity that covers the requirement. Dimensions, total capacity, and published inflation times are shown so the result can go straight into a mobilisation plan.
Assumptions and limits
- Seawater is taken as 1,025 kg/m³, freshwater as 1,000 kg/m³.
- The depth-to-pressure conversion uses the standard 10 m ≈ 1 bar approximation.
- Bags are assumed inflated at working depth; open-bottom parachute bags self-equalise, enclosed bags hold their shape with automatic pressure-relief valves set at 15–25 kPa.
- Residual buoyancy from the object’s own volume is optional — leave it at zero for a conservative plan.
- The combination search is capped at four bags (200 t). Heavier lifts are engineered systems, not bag counts.
From pre-plan to lift plan
These figures size the concept — they are not a lift plan. Send the calculation to our engineering team via WhatsApp or the quote request and you will receive a project-specific proposal: bag combination, rigging and attachment detail, inflation setup with control panel, and a drawing, together with a quotation. Every bag ships with repair kit, shackles, and certified webbing rated to a nominal 7:1 safety factor.