How To Calculate Buoyant Force

Buoyancy, or buoyant force, is based on Archimedes' Principle. This principle states, "Any object, wholly or partly immersed in a fluid, is buoyed up by a force equal to the weight of the fluid displaced by the object." Archimides' Principle is important in hydro-engineering applications, such as shipbuilding. The steps below detail how to calculate buoyant force.

Step 1

Obtain the volume of the object on which you wish to calculate the bouyant force. We will call this value for the volume "V."

Step 2

Determine what percentage (based on volume) of the object will be submerged in the water.

Step 3

Convert this percentage to a decimal number. We will call this value "v." For example; if 100 percent of the object is to be submerged, v= 1.0 . If 50 percent of the object is to be submersed, v= 0.50 .

Step 4

Substitute this value into the equation for buoyant force: FB = (V)(v) x (SPH2O) Where FB = buoyant force and SPH2O = Specific gravity of water (considered constant at 62.4 lb per cubic foot).

Step 5

Multiply V by v, and then multiply by 62.4 in order to get the value for the buoyant force, expressed in pounds. Consider the following example of a 2-foot by 2-foot by 2 foot cube to be submersed 25 percent of the way in water. V = 222 = 8 cubic feet v = 25% = 0.25 SPH2O = 62.4 pounds per cubic foot FB = 8 0.25 62.4 = 124.8 pounds

That's how you calculate buoyant force, and it helps illustrate how those large ships made of steel can float. That's quite a bit of force needed to push that 2 foot cube just one fourth of the way in the water!