Buoyancy Calculator
This buoyancy calculator finds the buoyant force on an object from the volume it displaces, the fluid density, and gravity, using Archimedes' principle F = ρ g V. Enter the values and the buoyancy calculator returns the buoyant force in newtons and kilograms-force. It covers fresh water, salt water, and other fluids.
Results
The Buoyancy Calculator is part of our Motion & Force collection. Browse more motion & force to size, convert, and check your work alongside this tool.
What the buoyancy calculator shows
Any object in a fluid is pushed up by a force equal to the weight of the fluid it displaces. The buoyancy calculator works out that upward force from the displaced volume and the fluid density. Compare it with the object's weight to see whether it floats, sinks, or hangs in place.
Flotation calculation with Archimedes' principle
A flotation calculation uses Archimedes' principle:
ρ is the fluid density, g gravity, and V the submerged volume. If the buoyant force is larger than the object's weight, it rises until only enough is submerged to balance its weight. That is why a ship floats with only part of its hull under water.
Buoyancy calculator example
An object displacing 0.01 m³ (10 litres) of fresh water. The buoyancy calculator gives:
So 10 litres of submerged volume can hold up 10 kg in fresh water.
How to do a buoyancy force calculation
For a buoyancy force calculation, enter the submerged volume in cubic metres, the fluid density, and gravity. Use 1,000 kg/m³ for fresh water and about 1,025 kg/m³ for sea water. The buoyancy calculator returns the force in newtons and in kgf, which is the mass the fluid can support.
Reading the buoyancy calculator pounds and kgf result
Kilograms-force is the easiest way to read buoyancy: each litre of fresh water displaced supports 1 kg. For a buoyancy calculator pounds figure, multiply kgf by 2.205. Subtract the object's own weight to get the extra load it can carry before sinking.
When to use the boat buoyancy calculator
- Sizing floats with a pontoon buoyancy calculator
- Checking a small boat's load with the boat buoyancy calculator
- Designing a buried tank that must not float
- Solving physics problems on floating and sinking
Common buoyancy calculator mistakes
- Using total volume when partly submerged. Only the submerged part counts.
- Wrong density. Sea water is denser than fresh water.
- Forgetting the object's weight. Net lift is buoyancy minus weight.
Tips for accurate results
- Convert litres to cubic metres by dividing by 1,000.
- Keep a safety margin for floats.
- Use local water density for precise work.
Pontoon buoyancy calculator and boat loading
A pontoon buoyancy calculator uses the full volume of each float as the maximum displaced volume. Multiply that volume in litres by the water density for the total lift in kilograms, subtract the weight of the structure, and keep a safety margin of at least 20 percent so the deck stays above water with waves and moving loads. The float volume itself can come from the tank volume calculator for cylindrical floats.
Tank buoyancy calculator for buried tanks
An empty buried tank in waterlogged ground can float out of the soil. A tank buoyancy calculator finds the uplift from the tank's full volume, which must be balanced by the tank's weight, the soil above, and anchors or concrete ballast.
Salt water buoyancy calculator reference
| Fluid | Density | Lift per litre |
|---|---|---|
| Fresh water | 1,000 kg/m³ | 1.00 kg |
| Sea water | 1,025 kg/m³ | 1.025 kg |
| Dead Sea water | 1,240 kg/m³ | 1.24 kg |
| Air | 1.225 kg/m³ | 0.0012 kg |
Buoyancy Calculator FAQ
What is the buoyancy formula?
Fb = ρ g V: fluid density times gravity times submerged volume.
How do I know if something floats?
It floats if its average density is lower than the fluid, or if the buoyant force when fully submerged exceeds its weight.
Why do things float better in salt water?
Salt water is denser, so the same volume displaced gives more lift.
How much weight can a pontoon hold?
Its displaced volume in litres gives the kilograms it can support in fresh water, minus its own weight.