How do you calculate zone of inhibition?

In order to measure the zone of inhibition we measure the diameter of the zone of inhibition including the diameter of the disc.

A Zone of Inhibition Test, also called a Kirby-Bauer Test, is a qualitative method used clinically to measure antibiotic resistance and industrially to test the ability of solids and textiles to inhibit microbial growth. The zone is measured by the diameter across the zone and you do not consider the disc at all. Zone is measured from edge to edge of the clear area , which means disc will be inside and included in the measurement.