How to use it
- Choose the fluid or enter a custom density in kg/m³.
- Enter the object’s volume and pick the unit (1 L = 0.001 m³).
- Give the object’s mass, or its density if you know the material.
- Read the buoyant force, whether it floats, and the part under water or the apparent weight.
Formulas
- Buoyant force (fully submerged): F = ρ fluid × V × g, with g = 9.80665 m/s².
- Weight: W = m × g. Object density: ρ = m ÷ V.
- Floating: submerged fraction = ρ object ÷ ρ fluid; at rest the buoyant force equals the weight.
- Sinking: apparent weight = W − F.
The fluid densities in the list are typical values; fresh water is rounded to 1000 kg/m³ and air is taken at sea level and 15 °C.
Worked example
A 2 L closed bottle weighing 1 kg in fresh water: F = 1000 × 0.002 × 9.80665 = 19.6133 N when pushed under, the weight is 9.80665 N, so it floats with 500 ÷ 1000 = 50% below the surface.
A 1 L block of steel (7850 kg/m³): mass 7.85 kg, weight 76.9822 N, buoyant force 9.80665 N, so it sinks and weighs 67.1756 N in the water.
Tips
- Volume, not size. Only the volume of fluid pushed aside counts, whatever the shape.
- Salt water lifts more. The same object floats a little higher in the sea than in a lake.
- Gases are fluids too. Choose air to see why a helium balloon rises: its total density, envelope included, must be below 1.225 kg/m³.