I can't seem to figure this simple helium balloon problem out? I would really appreciate some help? Thanks! Estimate the volume of a helium-filled balloon at STP if it is to lift a payload of 500 kg. The density of air is 1.29 kg/cubic meter and helium has a density of .178 kg/cubic meter. I think you use a formula like force bouyancy equals density of air times volume times gravity????????? but not quite sure.