http://en.wikipedia.org/wiki/Buoyancy
It is common to define a buoyant mass mb that represents the effective mass of the object with respect to gravity
where m(o) is the true (vacuum) mass of the object, whereas ρ(o) and ρ(f )are the average densities of the object and the surrounding fluid, respectively. Thus, if the two densities are equal, ρ(o) = ρ(f), the object appears to be weightless. If the fluid density is greater than the average density of the object, the object floats; if less, the object sinks.