Buoyancy is the upward force a fluid exerts on an immersed object. Archimedes’ principle states that this force equals the weight of the displaced fluid.

This is why the same object can behave differently in freshwater, seawater or a denser liquid.

Choose a fluid preset or enter a custom fluid density. Enter displaced volume, object mass and gravity. The result compares buoyant force with object weight and reports whether the entered state is positively buoyant, approximately neutral, or negatively buoyant.

Multiply fluid density by displaced volume and gravitational acceleration. For example, 0.01 m³ completely submerged in freshwater near 1000 kg/m³ displaces roughly 10 kg of water and experiences about 98 N of upward buoyant force under Earth gravity.

For a freely floating uniform object, the key comparison is average object density versus fluid density.

• Object density < fluid density: it can float.
• Object density ≈ fluid density: neutral buoyancy is possible.
• Object density > fluid density: it tends to sink unless another force acts.

For a floating object in static equilibrium, the fraction of its volume submerged is approximately the ratio of object average density to fluid density, provided the object is not taking on water and surface-tension effects are negligible.

Seawater is denser than freshwater, so the same displaced volume generally produces a larger buoyant force. This is why boats sit slightly higher and swimmers often feel more buoyant in saltwater.

Apparent weight is the downward weight force minus the upward buoyant force. Net buoyancy uses the same force difference with sign convention stated explicitly. These are useful for submerged equipment, ballast studies and educational physics examples.

A floating vessel must displace a fluid weight equal to its total weight. Boat and ballast design also depends on stability, center of gravity, hull form, free surface effects, reserve buoyancy and regulations. A simple buoyancy equation is therefore only the first layer of a real design problem.

Buoyancy calculators can explain the physics, but they should not be used to prescribe personal flotation devices, diving weights or safe dive depth. Certified equipment ratings, body composition, exposure protection, cylinder configuration and training procedures matter.

A sealed object displaces 0.020 m³ of freshwater. At 998 kg/m³ and standard gravity, buoyant force is roughly 998×0.020×9.80665 ≈ 195.7 N. If the object mass is 15 kg, its weight force is about 147.1 N, so net buoyancy is positive by roughly 48.6 N. That does not automatically prove stable floating orientation; stability is a separate question.

A direct laboratory method can compare an object’s apparent weight in air with its apparent weight while submerged. The difference corresponds to buoyant force under controlled conditions. For floating objects, displaced volume can also be inferred from equilibrium if total weight and fluid density are known. Surface tension and trapped air matter for very small samples.