The present invention relates to a method for determining activity of a catalyst in a polymerisation process, said polymerisation process comprising the steps of feeding a catalyst and a diluent into a storage vessel, to form a concentrated settled catalyst, feeding said concentrated settled catalyst into a mixing vessel equipped with mixing means and feeding hydrocarbon diluent into said mixing vessel, to form a diluted catalyst slurry, feeding said diluted catalyst slurry into a polymerisation reactor via a volumetric pump, and feeding at least one monomer into said polymerisation reactor to form a polymer. The method is characterised in that density of the diluted catalyst slurry is measured between an outlet of the mixing vessel and the volumetric pump, the amount of catalyst fed into the polymerisation reactor is determined based on the measured density of the diluted catalyst slurry, and the amount of catalyst fed into the polymerisation reactor is compared to the amount of the polymer formed to determine the activity of the catalyst. The invention also relates to a system suitable for carrying out the method.