Protostars ∼0.1 Myr old are heavily obscured, but their circumstellar dust discs can be studied by millimetre interferometry that resolves out the obscuring envelope. Consistent estimates are made for the disc masses of Class 0 protostars, and these range over 7–660 MJup. A simple grain coagulation model reproduces the mass distributions of more evolved discs around Class I protostars and Class II T Tauri stars (at ≲1, ≲5 Myr), implying that their observed dust is remnant material. The conversion of most dust grains into planetesimals thus occurs very early, at ∼0.1 Myr. As this is concurrent with the formation of the star itself, much of the disc is expected to accrete on to the central object, and a correlation of bolometric luminosity and disc mass is observed that agrees well with disc models. The observed Class 0 discs all contain upwards of 20 M of dust, allowing for the formation of ‘super-Earths’ around many Sun-like stars. Only 10 per cent of the dust mass needs to be converted into planetary cores to match the range of such core masses presently known.