The fundamental error associated to the sampling of heterogeneous particles is unavoidable and can only be minimized by an adequate selection of the sample mass and/or size reduction of the sample before division. An equation is derived to estimate the relative standard deviation of the fundamental error due to the sampling of low grade ores. The results of the application of the proposed equation are similar to the results of the Gy's formula for high and medium grade ores but are different for low grade ores. The main difference between the Gy's formula and the proposed equation for low grade gold ores is related to the liberation factor. (C) 2013 Elsevier B.V. All rights reserved.