Geochemical analysis of geological materials introduces errors at virtually every stage of sample preparation and analysis. Determining the actual analytical error (that error introduced during the analysis of prepared sub-samples of geological materials) is commonly difficult because many forms of analysis destroy the sub-sample. As a result, duplicate analysis cannot be undertaken to measure analytical error directly, and analytical error cannot be isolated from sub-sampling error. However, using replicate analyses of sub-samples of two different masses, and solving a system of three equations in three unknowns, the actual ‘analytical’ error can be deduced and distinguished from the sub-sampling error. This provides a means to estimate sub-sampling and analytical error magnitudes and to determine whether increasing sub-sample mass will result in an efficient reduction in overall error in geochemical analyses. It also provides a means to quantify sub-sampling error in reference materials so that they can be properly used in geochemical analysis to monitor and quantify analytical error.