If normal rate and depth of breathing means average for the average adult, normal is easy to define. But your normal depends on your weight, age and sex. Regular breathing moves a volume of air--called your tidal volume, or Vt--in and out with every breath. Vt is expressed in milliliters, or mL. Breathing rate, or frequency, F, is expressed as breaths per minute. Because your body at rest tries to maintain normal, overall breathing volumes every minute, if Vt changes, F can change proportionately in the opposite direction, within normal limits.

According to vital sign data published by the University of Virginia Health System, your breathing rate is normal at 15 to 20 breaths per minute for an average adult at rest, and abnormal above 25 or below 12. Normal adult Vt is 500mL per breath. Anesthesiologists at the University of Wisconsin estimate normal Vt on the relationship between body weight and air volume: Vt = 6 to 8 mL/kg in infants, and 7 mL/kg in adults. Kg = kilogram, which is 2.2 lbs.

Respiratory Institute researchers at the Cleveland Clinic developed formulas for normal Vt for infants, children and adults. These are used to determine volumes delivered by mechanical respirators for patients unable to breathe on their own. For adults between 80 and 250 lbs, breathing at F = 8 to 18: Males: Vt = 466.969 + (2.4 x lb) - (26.342 x F) Females: Vt = 456.408 + (1.794 x lb) - (22.716 x F) If Vt is known, these formulas can be solved for normal F at that volume.

Babies and youngsters, defined by the Respiratory Institute researchers as those between 10 and 79 lbs., have normally higher respiratory rates than adults, with F = eight to 25 breaths per minute. Calculating normal Vt does not depend on sex, as differences in physique are not pronounced yet at this stage of life. Vt = 14.137 + (3.470 x lb) - (6.861 x F)

The Cleveland Clinic researchers define infants for the purpose of calculating normal Vt and F as weighing between 6 and 9 lbs. Infants' normal respiratory rate is variable, but is usually between 30 and 50 breaths per minute. Vt = 7.160 + (2.820 x lb) - (0.265 x F)

Respiratory physiologists at Eastern Kentucky University point out that the depth and rate of breathing respond to the body's need to absorb oxygen and eliminate carbon dioxide. The degree to which rate and volume can increase also depend on a person's athletic conditioning and biochemistry. Normal values vary widely and can not be predicted with high probability.