Quantification of expiratory flow limitation during exercise has been demonstrated using computerized breath-by-breath analysis of the flow-volume curve. One of the parameters used in quantitation of airflow limitation is the rectangular area ratio (RAR) described in this journal by Ma et al. (2010). Upon closer review of the formula utilized in this paper, it is noted that the formula does not accurately capture the RAR, and the formula may underestimate the true RAR due to errors in various terms contained within the numerator and the denominator. A correct version of the formula is presented in this report. The correct formula does not require re-measurement of new variables but uses the existing variables in the correct mathematical and geometric context. This study has implications for validity of the data contained in the original paper as well as several other studies utilizing the same formula that have been published since the original publication. All future research using the RAR should use the correct formula, as contained in this paper.