How this calculation works
Use the submerged volume fraction for the current force and compare the entire available displacement capacity with the amount needed to balance weight. Current submergence need not be the equilibrium state.
Buoyancy = fluid density × gravity × displaced volume; balancing displacement = mass / fluid densityA worked example
An entered 1,000 kg/m³ fluid displaces 0.005 m³ at 50% of a 0.01 m³ capacity. At g = 9.80665 m/s², buoyancy is 49.03325 N. A 4 kg object weighs 39.2266 N and would need 0.004 m³ to balance its weight.
Good to know
Rigid displacement capacity in a uniform stationary fluid. No floating stability, orientation, flooding, trapped-air compression, surface tension, drag or safe load determination.
Reference: Reference: model, definitions and assumptions (opens in a new tab) ↗
The current submerged volume need not be an equilibrium
At the default 50% of 0.01 m³, the displaced volume is 0.005 m³. In the entered 1,000 kg/m³ fluid that displaces 5 kg of fluid. With gravity 9.80665 m/s², the buoyant force is 49.03325 N.
The entered 4 kg object weighs 39.2266 N, so buoyancy minus weight is +9.80665 N at that instant before other forces. An upward net force does not mean the object remains at the entered 50% submergence. That fraction is a supplied state, not an automatically solved floating equilibrium.
The density calculator checks mass per volume. Buoyancy instead uses displaced fluid mass, which need not equal the object’s own mass.
Required displacement can be compared with the available volume
Balancing the default 4 kg weight in a 1,000 kg/m³ fluid requires 0.004 m³ of displaced fluid. That is 40% of the supplied 0.01 m³ capacity. The gravity factor cancels in this weight-versus-buoyancy balance when both use the same entered gravity.
An 11 kg object with the same external capacity would need 0.011 m³. Even full submergence cannot supply that amount, so the required 110% fraction stays visible and the capacity check reports insufficient displacement. A ten-kilogram object exactly matches the full-volume displaced fluid mass in this ideal model.
A volume fraction does not generally equal a height fraction
Half the height of an irregular shape need not contain half its volume. The submergence input is explicitly a volume percentage. External displacement capacity may also differ from solid material volume for a sealed hollow object; the entered capacity must describe the object and situation you intend.
The hydrostatic pressure calculator explains a separate depth-pressure relation, while the density calculator checks mass per volume. This buoyancy page does not determine floating stability, orientation, flooding, trapped-air compression or a safe load. Enough displacement to balance weight is only one requirement for a real floating configuration.
A few questions, answered
Is the entered submerged percentage solved automatically?
No. It specifies the current displaced volume. The required fraction for ideal weight balance is reported separately.
Can the required fraction exceed 100%?
Yes. That means the supplied displacement capacity cannot balance the weight even when fully submerged under this model.
Does buoyancy disappear when an object sinks?
No. The model can show buoyancy smaller than weight, leaving a downward net force before other forces.
Does an equilibrium-capacity result prove stable floating?
No. Stability, orientation, flooding and other effects are outside the calculation.