Lift is the difference between mass of the liquid (gas) displaced and mass of the object (or buoyancy minus mass of the object itself). So, volume V of hydrogen displaces V air and lift is V(dair-dhydrogen). Density of air at STP is around 1.29 g/L, density of hydrogen at STP - around 0.089 g/L. That gives lift of 1.2 g/L, you need 1kg, so 830 L will do. That's neglecting mass of the baloon.
Densities can be easily calculated from pV=nRT and n=mass/molar mass, or even simpler from d = molar mass/22.4.