[Radical Edward]

the density of helium is

0.1786 g/L

and air is

1.2 g/L

so the difference is about 1g/l.

work out the mass of the thing you want to lift in grams, and then you need about that many litres of (pure) helium - you also have to include the container of course in the mass calculations, since you need tp pick that up too..

I did this while drunk, picking up wine glasses with party balloons - though admittedly the helium was probably not very pure. it took a suprisingly large number, actually.