The hysteresis in magnetisation is extensively used to infer the critical current density of high Tc superconductors. We use calculated magnetisation curves, obtained analytically for two particular forms of Jc( H), to examine the validity of the method currently used. We show that this method is valid only at large fields, and that the region of validity depends on the sample size. We compare our results with existing data. We also examine the case of a general form of Jc( H), and present a method for obtaining a zero field Jc from magnetisation curves.