Buoyancy Calculator
Calculate the buoyant force on an object using Archimedes' principle F = ρVg. Enter displaced volume, fluid type, and optionally the object mass to determine whether the object floats or sinks. No signup, runs entirely in your browser.
How to Use the Buoyancy Calculator
- 1Choose a fluid — freshwater, seawater, mercury, oil, or air — or enter a custom density in kg/m³.
- 2Enter the displaced volume in cubic meters. Remember 1 liter = 0.001 m³.
- 3Optionally enter the object's mass to compare buoyant force against weight and get a float-or-sink verdict.
- 4Read the buoyant force in Newtons, computed from Archimedes' principle F = ρVg.
Worked Example: Will a 40 kg Crate Float?
A sealed crate has a volume of 0.05 m³ (50 liters) and a mass of 40 kg. Fully submerged in freshwater, it displaces 0.05 m³, so the buoyant force is F = 1000 × 0.05 × 9.81 = 490.5 N. Its weight is 40 × 9.81 = 392.4 N. Buoyancy exceeds weight, so the crate floats. The shortcut check gives the same answer: average density = 40 / 0.05 = 800 kg/m³, which is less than water's 1000 kg/m³.
Once floating at the surface, the crate no longer displaces its full volume — it settles until the displaced water weighs exactly 40 kg, i.e. 0.04 m³. That means 0.04 / 0.05 = 80% of the crate sits below the waterline. Repeat the calculation with seawater (1025 kg/m³) and the displaced volume drops to 40 / 1025 ≈ 0.039 m³ — about 78% submerged, which is why the same object rides slightly higher in the ocean than in a lake.
Buoyancy Physics Tips
Average density check
An object floats if its average density is less than the fluid density. Calculate: object density = mass / volume, then compare to the fluid's density (e.g. 1000 kg/m³ for water).
Partial submersion
A floating object only displaces fluid equal to its weight. If a 1 kg block floats in water, it displaces exactly 0.001 m³ (1 liter) regardless of its total volume.
Mercury is dense
Mercury at 13,600 kg/m³ is 13.6 times denser than water. Even dense metals like lead (11,340 kg/m³) float in mercury — which is why early thermometers could use mercury to support a column.
Air buoyancy
Air has density 1.225 kg/m³. A 1 m³ balloon displaces ~1.225 N of air buoyancy. For a helium balloon to lift, the buoyancy must exceed the total weight of the gas plus envelope.
Frequently Asked Questions
What is Archimedes' principle?
Archimedes' principle states that the buoyant force on an object submerged in a fluid equals the weight of the fluid displaced by the object: F_b = ρ × V × g, where ρ is fluid density, V is displaced volume, and g is gravitational acceleration.
When does an object float vs sink?
An object floats when the buoyant force equals or exceeds its weight (F_b ≥ mg). It sinks when gravity exceeds buoyancy. A hollow steel ship floats because its average density — hull plus enclosed air — is less than water's density.
What is neutral buoyancy?
Neutral buoyancy occurs when buoyant force exactly equals object weight. Submarines achieve neutral buoyancy by adjusting ballast tanks. It allows them to hover at a fixed depth without using engine power.
What units should I use for volume?
Enter volume in cubic meters (m³). Common conversions: 1 liter = 0.001 m³, 1 cubic centimeter = 10⁻⁶ m³. A 1-liter bottle fully submerged in water displaces 0.001 m³ and experiences ~9.81 N of buoyant force.
Why is buoyancy greater in seawater than freshwater?
Seawater has a density of ~1025 kg/m³ vs ~1000 kg/m³ for freshwater. The extra 2.5% density means a 2.5% greater buoyant force. This is why it is easier to float in the ocean than in a freshwater lake.
Is my data stored?
No. All calculations run locally in your browser. No data is sent to any server.