This lung capacity calculator determines all respiratory capacities based on lung volumes from vital capacity to functional residual capacity and TLC. There is in depth information on the volumes that need to be input and the formulas used below the form.

Inspiratory reserve volume:
Tidal volume:
Expiratory reserve volume:
Residual volume:

## How does the lung capacity calculator work?

This health tool determines all four of the pulmonary capacities based on the respiratory volumes extracted after spirometry.

The lung capacity calculator requires the following:

■ Inspiratory reserve volume – is approximately 3 L and represents the amount of air that can be forcefully inspired after a normal inspiration.

■ Tidal volume – is approximately 0.5 L and represents the volume of air which is circulated through inhalation and expiration during a normal respiration.

■ Expiratory reserve volume – is approximately 1.2 L and represents the volume of air which can be exhaled forcefully after a normal expiration.

■ Residual volume – is approximately 1.2 L and represents the amount of air that remains in the lungs.

The difference between lung volumes and lung capacities is that the former are directly measured while the latter are inferred from volumes.

The formulas used for the lung capacities are:

Vital capacity (VC) = Inspiratory reserve volume (IRV) + Tidal volume (TV) + Expiratory reserve volume (ERV)

Inspiratory capacity (IC) = Inspiratory reserve volume (IRV) + Tidal volume (TV)

Functional residual capacity (FRC) = Expiratory reserve volume (ERV) + Residual volume (RV)

■ Total lung capacity (TLC) = Inspiratory reserve volume (IRV) + Tidal volume (TV) + Expiratory reserve volume (ERV) + Residual volume (RV)

The following table introduces the average normal values for lung capacities for men and women:

 Lung capacity Formula Men (L) Women (L) Vital capacity IRV + TV + ERV 4.6 3.1 Inspiratory capacity IRV + TV 3.5 2.4 Functional residual capacity ERV + RV 2.3 1.8 Total lung capacity IRV + TV + ERV + RV 5.8 4.2

Restrictive pulmonary diseases decrease lung volumes therefore decrease pulmonary capacities as well. Some examples of restrictive conditions include pulmonary fibrosis or pneumothorax.

Obstructive pulmonary diseases, such as asthma or COPD lead to some increases in lung volumes.

There are also some natural occurring changes in lung volumes, such as those characteristic to pregnancy, when functional residual capacity drops by almost 20% due to the uterus compressing the diaphragm and the thoracic cavity. On the other hand, tidal volume increases in order to meet the ventilation requirement.

In general, taller individuals, those who live at higher altitudes and those with normal or subnormal weights tend to have larger lung volumes compared to those smaller, living at lower altitudes or suffering from obesity.

## Example of a lung capacity calculation

Let’s take the following results from functional lung tests:

■ Inspiratory reserve volume = 3 L;

■ Tidal volume = 0.5 L;

■ Expiratory reserve volume = 1.3 L;

■ Residual volume = 1.2 L.

The resulting figures are:

■ Vital capacity (VC) = 4.8 L;

■ Inspiratory capacity (IC) = 3.5 L;

■ Functional residual capacity (FRC) = 2.5 L;

■ Total lung capacity (TLC) = 6 L.

## References

1) Flesch JD, Dine CJ. (2012) Lung volumes: measurement, clinical use, and coding. Chest; 142(2):506-10.

2) Jones RL, Nzekwu MM. (2006) The effects of body mass index on lung volumes. Chest; 130(3):827-33.

3) Otis EO. (1893) Measurements of the Chest and Lung Capacity. Trans Am Climatol Assoc; 10: 199–202.

4) Ruppel GL. (2012) What is the clinical value of lung volumes? Respir Care; 57(1):26-35.

21 Sep, 2016