Quantitaion of actual and expected complete compensation for chronic hypercapnia in man is facilitated by addition of data of Brackett et al. in the form of an are to the Siggaard-Andersen alignment nomogram. Perecntage of compensation is defined as 100 times the ratio of observed to predicted (fully compensated) base excess at the patient's Pco, Extracellular fluid base excess, defined as BE3is estimated from a 3-g hemoglobin line added to the nomogram.