Archimedes reportedly worked out buoyancy in his bathtub over 2,200 years ago, and the same principle still explains everything from why a rubber duck floats to why a 200,000-ton cargo ship doesn't sink. This calculator applies that one formula — buoyant force equals the weight of displaced fluid — to three everyday questions: how much lift a submerged volume gets, whether an object floats or sinks, and how much of a floating object stays underwater.
How the Buoyancy Calculator works
Every calculation here comes from Archimedes' principle: Fb = ρfluid × V × g. The Buoyant Force tab plugs your fluid density and displaced volume directly into that formula. The Float or Sink tab skips the volume and gravity terms entirely — since they cancel out when comparing an object to the fluid around it, only the density ratio decides the outcome. The Fraction Submerged tab uses that same density ratio to find the equilibrium point where a floating object's weight exactly balances the buoyant force pushing up on its submerged portion.
Inputs and what they mean
Fluid density (kg/m³) is the property of the liquid or gas the object sits in — fresh water (1000), seawater (1025, denser from dissolved salt), or specialty fluids like mercury (13,546) for lab equipment. Displaced volume (m³) is the submerged portion of the object's volume, not necessarily its total volume — a partly floating object only displaces its underwater section. Object density, when you provide it, is mass divided by total volume; get an unknown object's density by weighing it and measuring its volume (or using the water-displacement method: submerge it and record how much the water level rises).
Limits and edge cases
This calculator assumes a uniform fluid at constant density and a rigid object that doesn't deform or absorb fluid — real seawater density varies with temperature and salinity, and porous materials like some woods can slowly waterlog and change density over time. It also treats "floats" as a simple density comparison; real floating stability (whether a ship tips over) depends on the shape and center of mass too, which is a separate engineering question from whether it floats at all. For anything beyond a quick estimate — naval architecture, industrial tank design — consult an engineer.