Inspiratory and expiratory ratio is a figure that shows the relationship between a person's inspiratory time and his expiratory time. Typically, the expiratory time will be greater than the inspiratory time because it takes a person longer to exhale than inhale. If this isn't, the inspiratory time is longer than the expiratory time, and the person may be experiencing gas trapping, which could lead to barotrauma or even cardiac arrest in extreme cases.

Lay down or sit in a comfortable position and set your stopwatch to 30 seconds.

Inhale and at the same time start your stop watch. Count how many times you take a breath within 30 seconds. As an example, you inhale 10 times in 30 seconds.

Multiply the number of times you inhale in 30 seconds by 2 to determine your inhalation rate in one minute. In the example, multiply 10 by 2. You inhale 20 times in 1 minute.

Subtract the inspiratory rate from 60 to determine the expiratory rate. In the example, subtract 20 from 60. You have an expiratory rate of 40 seconds in 1 minute.

Write the inspiratory and expiratory rate as a ratio. List the inspiratory rate first. In the example, the ratio would look like this: 20:40.

Reduce the ratio by dividing each number by the greatest common factor shared by the two numbers. In the example, the shared greatest common factor is twenty. Dividing 20 by 20 gives you 1. Dividing 40 by 20 gives you 2. The inspiratory and expiratory ratio is 1:2.