Buoyant Force Calculator

About the Buoyant Force Calculator

Buoyant force is the upward force a fluid exerts on an immersed object, and its magnitude equals the weight of the displaced fluid. That relationship is the core of Archimedes' principle and explains why some objects float while others sink.

This calculator works with several common shapes, or a custom volume, and it can also handle partial submersion. That makes it useful for pontoons, floats, hull sections, balloons, anchors, and any situation where only part of the object is in the fluid.

The comparison table shows how the same object behaves in different fluids, from air to mercury, so the effect of density is easy to see at a glance.

When This Page Helps

Buoyancy problems are mostly geometry plus fluid density, but those pieces become tedious to combine by hand once the object shape or submersion changes. Putting the shapes, fluids, and comparison table together keeps the calculation easier to reason about.

How to Use the Inputs

1. Select the object shape and enter its dimensions.
2. Choose a fluid from the dropdown or enter a custom density.
3. Set the percentage of the object that is submerged (100% = fully submerged).
4. Read the buoyant force, displaced mass, and weight supported.
5. Compare the same object across different fluids in the table.

Example Calculation

Tips & Best Practices

• Buoyant force depends only on fluid density and displaced volume — not on object mass.
• A 1-liter object fully submerged in water experiences about 9.8 N of buoyancy.
• Mercury buoyancy is about 13.5× that of water for the same object.
• Partially submerged objects reach equilibrium when buoyant force equals weight.
• For pontoon design, use 50-75% submersion to leave safety margin.

Engineering Applications

Buoyant force calculations are critical in offshore engineering (oil platform design), marine engineering (hull and ballast design), civil engineering (bridge pontoons), and aerospace (lighter-than-air craft). The oil industry uses buoyancy to design tension-leg platforms and spar buoys that float at engineered depths.

Buoyancy in Everyday Life

Life jackets work by adding buoyant volume to the human body, lowering average density below water. Swimming pool floating aids, inflatable rafts, and fishing bobbers all exploit buoyancy. Even cooking — testing egg freshness by floating in water — uses buoyancy principles.

Scale Effects

Buoyant force scales with the cube of linear dimensions (since volume ∝ length³). Doubling an object's size increases buoyant force eightfold. This is why large ships can carry enormous loads — the displaced water volume grows much faster than the hull weight.

Frequently Asked Questions

• What is buoyant force?

  Buoyant force is the upward push a fluid exerts on any object placed in it. It equals the weight of the displaced fluid, as stated by Archimedes' principle: F_b = ρ_fluid × V_displaced × g.

• Does shape affect buoyant force?

  Shape determines volume, which determines the amount of displaced fluid. For the same volume, shape does not matter — only the displaced fluid volume determines buoyant force.

• Why use partial submersion?

  Many real-world objects are only partially submerged — boats, floating platforms, buoys, and ice. The partial submersion setting lets you calculate buoyant force for the actual immersed portion.

• What is the buoyant force of air?

  Air has a density of only 1.225 kg/m³ at sea level, so buoyant force in air is very small. However, for light objects like helium balloons, this small force is enough to cause them to rise.

• How do I find the maximum load a float can carry?

  Set submersion to 100% and read the "Weight Supported" value. This is the maximum total mass (float + cargo) that can be supported before the object is fully submerged.

• Why is buoyant force in mercury so large?

  Mercury has a density of 13,546 kg/m³ — about 13.5 times denser than water. Since buoyant force is proportional to fluid density, the same object experiences 13.5× more buoyancy in mercury.