Johns Hopkins now has a free online pulmonary function calculator that computes predicted values for some of the most common breathing measurements. Predicted values are simply the amounts that medical professionals would expect, based on basic individual data–in this case, a person’s age, height, and gender. They are extrapolated from broad demographic studies; you will notice that the Johns Hopkins site actually includes multiple possible values, based on the various separate studies.
Two particular measurements, vital capacity and FEV1 (forced expiratory volume in 1 second), are of special interest to brass musicians, as discussed in articles elsewhere on this site. They can be measured relatively easily on your own with a pocket spirometer and a peak flow/FEV1 meter. Predicted values for these measurements simply provide a means for comparing your breathing measurements with those of the average person of your same age, height, and gender.