The Respiration rate is the number of breaths a person takes per minute (more formally, the number of movements indicative of inspiration and expiration per unit time). The unit of measure is expressed in breaths per minute (BPM). The rate is usually measured when a person is at rest and simply involves counting the number of breaths for one minute by counting how many times the chest rises.
The aim of measuring respiratory rate is to determine whether the respirations are normal, abnormally fast (tachypnea), abnormally slow (bradypnea), or nonexistent (apnea). Respiration rates may increase with fever, illness, and with other medical conditions. When checking respiration, it is important to also note whether a person has any difficulty breathing.
Normal respiration rates for an adult person at rest range from 12 to 16 breaths per minute.
Source: John Hopkins Institute