Archimedes’ principle of buoyancy tells us that objects submerged in a fluid experience a buoyant force equal to the mass of the fluid displaced. Let’s assume an average human weight of 80kg. The average density of humans near to that of water, being about 1g/cm³. Therefore, our 80kg human has a volume of about 80 litres, or 0.08m³. Given that the density of air at sea level is 1.225kg/m³, we can calculate the displaced volume of air to weigh 1.225*0.08 = 98g. This is how much lighter the human would weigh in a vacuum!