If the object involved sinks, it displaces a volume, V, of the fluid equal to the object's volume, V, when on dry land.
If the object floats, it displaces a volume, V, of the fluid equal only to as much as is under the surface. That volume can be hard to determine depending on the shape of the object.
In either case, the buoyant force, #F_b#, equals the volume, V; times the density, #rho# (in #(kg)/m^3#) of the fluid; times the value of g.
#F_b = V*rho*g#
I hope this helps,