Phase angle is a measure obtained from the relation between the direct measures of resistance and reactance derived from an individual’s BIA reading. It is calculated using the following formula:

Phase angle = arc – tangent reactance / resistance x 180º / π

Phase angle gives a picture of overall health, in particular cell integrity and quality. It has also been used to predict body cell mass and has been used as a nutritional indicator in adults and children. A low phase angle is consistent with an inability of cells to store energy and an indication of breakdown in the selective permeability of cellular membranes. A high phase angle is consistent with large quantities of intact cell membranes and body cell mass.

An individual’s phase angle reading naturally decreases with age. Usually, a phase angle of six or greater is desired for men and five or greater for women. Fit adolescents may have a phase angle greater than 10.

Aside from very old age, a very low phase angle is consistent with malnutrition, infection (such as HIV or AIDS), chronic disease (for example cirrhosis, renal disease or pulmonary tuberculosis), an abusive lifestyle (chronic alcoholism) and most types of cancer.

Phase angle can also be used in sports medicine as a low phase angle can also indicate extreme physical exhaustion, typical in athletes who have undergone extreme sports tours for example.