How much helium does it take to lift a 2 pound weight?

Answer 1

Assume we're surrounded by air at standard temperature and pressure, and the helium is also at S.T.P.

The weight of a helium balloon of volume V would be equal to the density of helium multiplied by V multiplied by g (the acceleration due to gravity), i.e. VgρHe, and the weight of air displaced is similarly Vgρair. By Archimedes' principle, the balloon experiences an upward force equal to the difference, Vg(ρair - ρHe). If you want to lift a mass m then we must have Vg(ρair - ρHe) > mg, which gives:

V > m / (ρair - ρHe).

Setting m to 2 lbs (which is 0.91 kg) and plugging in the S.T.P. densities for air (typically 1.2 kg/m3) and helium (0.18 kg/m3) you get:

V > 0.89 m3,

i.e you need at least 890 litres (32 cubic feet) of helium at standard temperature and pressure.