Long-term weathering rates are determined by comparing the chemistry and mineralogy of soil profiles of known age and the fresh source rock composition. Such calculations are based on enrichment/depletion factors determined using immobile element contents, usually Ti or Zr. For convenience, both positive and negative changes are expressed relative to a representative elementary volume. This method has been applied to several investigation areas on silicate host material. The calculation of time-averaged weathering rates of a soil profile or a corresponding soil horizon is realized with a mathematical procedure described in literature. However, this procedure contains an error that may substantially influence the results. This error is caused by an incorrect use of soil and protolith height. An amended version of the calculation procedure is presented. An example is given to demonstrate the differences between the old and the revised procedure.